fix typeguard warnings in StructRef tests
fix typeguard warnings in StructRef tests

This adresses a latent error in in the StructRef tests that only
manifests when running under typeguard.

```
[2021-03-03 08:34:48,491] {docker_operator.py:262} INFO - ======================================================================
[2021-03-03 08:34:48,491] {docker_operator.py:262} INFO - ERROR: test_structref_caching (numba.tests.test_struct_ref.TestStructRefCaching)
[2021-03-03 08:34:48,491] {docker_operator.py:262} INFO - ----------------------------------------------------------------------
[2021-03-03 08:34:48,492] {docker_operator.py:262} INFO - Traceback (most recent call last):
[2021-03-03 08:34:48,492] {docker_operator.py:262} INFO - File "/src/numba/tests/test_struct_ref.py", line 318, in test_structref_caching
[2021-03-03 08:34:48,492] {docker_operator.py:262} INFO - check(cached=False)
[2021-03-03 08:34:48,492] {docker_operator.py:262} INFO - File "/src/numba/tests/test_struct_ref.py", line 306, in check
[2021-03-03 08:34:48,492] {docker_operator.py:262} INFO - st = check_make(vs, ctr)
[2021-03-03 08:34:48,492] {docker_operator.py:262} INFO - File "/src/numba/core/dispatcher.py", line 440, in _compile_for_args
[2021-03-03 08:34:48,493] {docker_operator.py:262} INFO - raise e
[2021-03-03 08:34:48,493] {docker_operator.py:262} INFO - File "/src/numba/core/dispatcher.py", line 373, in _compile_for_args
[2021-03-03 08:34:48,493] {docker_operator.py:262} INFO - return_val = self.compile(tuple(argtypes))
[2021-03-03 08:34:48,493] {docker_operator.py:262} INFO - File "/src/numba/core/dispatcher.py", line 911, in compile
[2021-03-03 08:34:48,493] {docker_operator.py:262} INFO - cres = self._compiler.compile(args, return_type)
[2021-03-03 08:34:48,493] {docker_operator.py:262} INFO - File "/src/numba/core/dispatcher.py", line 80, in compile
[2021-03-03 08:34:48,493] {docker_operator.py:262} INFO - status, retval = self._compile_cached(args, return_type)
[2021-03-03 08:34:48,494] {docker_operator.py:262} INFO - File "/src/numba/core/dispatcher.py", line 94, in _compile_cached
[2021-03-03 08:34:48,494] {docker_operator.py:262} INFO - retval = self._compile_core(args, return_type)
[2021-03-03 08:34:48,494] {docker_operator.py:262} INFO - File "/src/numba/core/dispatcher.py", line 107, in _compile_core
[2021-03-03 08:34:48,494] {docker_operator.py:262} INFO - cres = compiler.compile_extra(self.targetdescr.typing_context,
[2021-03-03 08:34:48,494] {docker_operator.py:262} INFO - File "/src/numba/core/compiler.py", line 608, in compile_extra
[2021-03-03 08:34:48,494] {docker_operator.py:262} INFO - return pipeline.compile_extra(func)
[2021-03-03 08:34:48,494] {docker_operator.py:262} INFO - File "/src/numba/core/compiler.py", line 354, in compile_extra
[2021-03-03 08:34:48,495] {docker_operator.py:262} INFO - return self._compile_bytecode()
[2021-03-03 08:34:48,495] {docker_operator.py:262} INFO - File "/src/numba/core/compiler.py", line 416, in _compile_bytecode
[2021-03-03 08:34:48,495] {docker_operator.py:262} INFO - return self._compile_core()
[2021-03-03 08:34:48,495] {docker_operator.py:262} INFO - File "/src/numba/core/compiler.py", line 396, in _compile_core
[2021-03-03 08:34:48,495] {docker_operator.py:262} INFO - raise e
[2021-03-03 08:34:48,495] {docker_operator.py:262} INFO - File "/src/numba/core/compiler.py", line 387, in _compile_core
[2021-03-03 08:34:48,496] {docker_operator.py:262} INFO - pm.run(self.state)
[2021-03-03 08:34:48,496] {docker_operator.py:262} INFO - File "/src/numba/core/compiler_machinery.py", line 339, in run
[2021-03-03 08:34:48,496] {docker_operator.py:262} INFO - raise patched_exception
[2021-03-03 08:34:48,496] {docker_operator.py:262} INFO - File "/src/numba/core/compiler_machinery.py", line 330, in run
[2021-03-03 08:34:48,496] {docker_operator.py:262} INFO - self._runPass(idx, pass_inst, state)
[2021-03-03 08:34:48,496] {docker_operator.py:262} INFO - File "/src/numba/core/compiler_lock.py", line 35, in _acquire_compile_lock
[2021-03-03 08:34:48,496] {docker_operator.py:262} INFO - return func(*args, **kwargs)
[2021-03-03 08:34:48,497] {docker_operator.py:262} INFO - File "/src/numba/core/compiler_machinery.py", line 289, in _runPass
[2021-03-03 08:34:48,497] {docker_operator.py:262} INFO - mutated |= check(pss.run_pass, internal_state)
[2021-03-03 08:34:48,497] {docker_operator.py:262} INFO - File "/src/numba/core/compiler_machinery.py", line 262, in check
[2021-03-03 08:34:48,497] {docker_operator.py:262} INFO - mangled = func(compiler_state)
[2021-03-03 08:34:48,497] {docker_operator.py:262} INFO - File "/src/numba/core/typed_passes.py", line 393, in run_pass
[2021-03-03 08:34:48,497] {docker_operator.py:262} INFO - lower.create_cpython_wrapper(flags.release_gil)
[2021-03-03 08:34:48,497] {docker_operator.py:262} INFO - File "/src/numba/core/lowering.py", line 242, in create_cpython_wrapper
[2021-03-03 08:34:48,498] {docker_operator.py:262} INFO - self.context.create_cpython_wrapper(self.library, self.fndesc,
[2021-03-03 08:34:48,498] {docker_operator.py:262} INFO - File "/src/numba/core/cpu.py", line 162, in create_cpython_wrapper
[2021-03-03 08:34:48,498] {docker_operator.py:262} INFO - builder.build()
[2021-03-03 08:34:48,498] {docker_operator.py:262} INFO - File "/src/numba/core/callwrapper.py", line 122, in build
[2021-03-03 08:34:48,498] {docker_operator.py:262} INFO - self.build_wrapper(api, builder, closure, args, kws)
[2021-03-03 08:34:48,498] {docker_operator.py:262} INFO - File "/src/numba/core/callwrapper.py", line 155, in build_wrapper
[2021-03-03 08:34:48,498] {docker_operator.py:262} INFO - val = cleanup_manager.add_arg(builder.load(obj), ty)
[2021-03-03 08:34:48,499] {docker_operator.py:262} INFO - File "/src/numba/core/callwrapper.py", line 30, in add_arg
[2021-03-03 08:34:48,499] {docker_operator.py:262} INFO - native = self.api.to_native_value(ty, obj)
[2021-03-03 08:34:48,499] {docker_operator.py:262} INFO - File "/src/numba/core/pythonapi.py", line 1390, in to_native_value
[2021-03-03 08:34:48,499] {docker_operator.py:262} INFO - return impl(typ, obj, c)
[2021-03-03 08:34:48,499] {docker_operator.py:262} INFO - File "/src/numba/core/boxing.py", line 445, in unbox_array
[2021-03-03 08:34:48,499] {docker_operator.py:262} INFO - nativearycls = c.context.make_array(typ)
[2021-03-03 08:34:48,499] {docker_operator.py:262} INFO - File "/src/numba/core/base.py", line 1001, in make_array
[2021-03-03 08:34:48,500] {docker_operator.py:262} INFO - return arrayobj.make_array(typ)
[2021-03-03 08:34:48,500] {docker_operator.py:262} INFO - File "/src/numba/np/arrayobj.py", line 68, in make_array
[2021-03-03 08:34:48,500] {docker_operator.py:262} INFO - class ArrayStruct(base):
[2021-03-03 08:34:48,500] {docker_operator.py:262} INFO - File "/opt/conda/envs/testenv_cdb4dfe6-77d1-4e76-bdad-0d927a493a21/lib/python3.8/site-packages/typeguard/__init__.py", line 890, in typechecked
[2021-03-03 08:34:48,500] {docker_operator.py:262} INFO - kwargs[name] = typechecked(
[2021-03-03 08:34:48,500] {docker_operator.py:262} INFO - File "/opt/conda/envs/testenv_cdb4dfe6-77d1-4e76-bdad-0d927a493a21/lib/python3.8/site-packages/typeguard/__init__.py", line 906, in typechecked
[2021-03-03 08:34:48,501] {docker_operator.py:262} INFO - warn('no type annotations present -- not typechecking {}'.format(function_name(func)))
[2021-03-03 08:34:48,501] {docker_operator.py:262} INFO - UserWarning: Failed in nopython mode pipeline (step: native lowering)
[2021-03-03 08:34:48,501] {docker_operator.py:262} INFO - no type annotations present -- not typechecking numba.np.arrayobj.make_array.<locals>.ArrayStruct.shape
[2021-03-03 08:34:48,501] {docker_operator.py:262} INFO -
[2021-03-03 08:34:48,501] {docker_operator.py:262} INFO - ----------------------------------------------------------------------
[2021-03-03 08:34:48,501] {docker_operator.py:262} INFO - Ran 10343 tests in 1559.663s
[2021-03-03 08:34:48,501] {docker_operator.py:262} INFO -
[2021-03-03 08:34:48,502] {docker_operator.py:262} INFO - FAILED (errors=1, skipped=1072, expected failures=12)
```

cpcloud/numba,gmarkall/numba,seibert/numba,seibert/numba,stuartarchibald/numba,IntelLabs/numba,IntelLabs/numba,stuartarchibald/numba,cpcloud/numba,gmarkall/numba,stonebig/numba,cpcloud/numba,gmarkall/numba,seibert/numba,IntelLabs/numba,IntelLabs/numba,gmarkall/numba,stonebig/numba,gmarkall/numba,stonebig/numba,stuartarchibald/numba,stonebig/numba,numba/numba,numba/numba,stonebig/numba,cpcloud/numba,cpcloud/numba,numba/numba,seibert/numba,stuartarchibald/numba,numba/numba,numba/numba,stuartarchibald/numba,IntelLabs/numba,seibert/numba