Add unit test for compatibility_checker.py

[liyanhui1228]

Closes #22

[brianquinlan]

I think that there is too much Mock usage here for these tests to be convincing. I'd say:

1. don't test check() or retrying_check()
2. mock retrying_check() to test get_...(). Don't mock ThreadPoolExecutor. Or maybe make the executor a constructor parameter and pass in a fake e.g.

class FakeExecutor:
def map(self, *args): return map(*args)

[liyanhui1228]

@brianquinlan Thanks for the suggestion! Updated to use FakeExecutor to test the functions, and mocked def retrying_checks.

[brianquinlan]

I would make this an optional constructor argument for CompatibilityChecker.

[liyanhui1228]

Done.