Bug-fix: We were loading a None test_spec_filename when TestSpec() ctor was called without parameters
Bug-fix: We were loading a None test_spec_filename when TestSpec() ctor was
called without parameters

Bug print: 'TestSpec::load('None') coercing to Unicode: need string or buffer,
NoneType found' when calling:
```python
def get_test_spec_from_yt_module(opts):
test_spec = TestSpec()
```

```
mbedgt -VS -n mbed-drivers-test-generic_tests
mbedgt: using 'module.json' from current directory!
TestSpec::load('None') coercing to Unicode: need string or buffer, NoneType found
mbedgt: checking for yotta target in current directory
        reason: no --target switch set
mbedgt: checking yotta target in current directory
        calling yotta: yotta --plain target
mbedgt: assuming default target as 'frdm-k64f-gcc'
```

ARMmbed/greentea