fix tests for big-endian platforms
fix tests for big-endian platforms

On powerpc and powerpc64 one test fails as:

```
  $ py.test -v
  ...
  test_umsgpack.py::TestUmsgpack::test_pack_ext_handler FAILED
  test_umsgpack.py::TestUmsgpack::test_unpack_ext_handler FAILED
  ...

  self = <test_umsgpack.TestUmsgpack testMethod=test_pack_ext_handler>

    def test_pack_ext_handler(self):
        for (name, obj, data) in ext_handlers_test_vectors:
            obj_repr = repr(obj)
            print("\tTesting %s: object %s" %
                  (name, obj_repr if len(obj_repr) < 24 else obj_repr[0:24] + "..."))
            packed = umsgpack.packb(obj, ext_handlers=ext_handlers)
  >           self.assertEqual(packed, data)
  E           AssertionError: b'\xd7 ?\x80\x00\x00@\x00\x00\x00' != b'\xd7 \x00\x00\x80?\x00\x00\x00@'

  test_umsgpack.py:484: AssertionError
```

The problem here is in 'struct.pack' output:
it uses native endianness format but test hardcodes little-endian output.

The change forces 'struct.pack' into little-endian format.
That way all tests pass:.

Signed-off-by: Sergei Trofimovich <110fdadc76b6bce16977f568bdb4723eaed6fb3f@gentoo.org>
Signed-off-by: Vanya A. Sergeev <7a38d8cbd20d9932ba948efaa364bb62651d5ad4@sergeev.io>

vsergeev/u-msgpack-python