test_util: Add test for get_dict_key_from_value

[sonic2kk]

Adds a test for the get_dict_key_from_value. It is written so it tests dicts with various different values, far more exotic than the couple of uses in the codebase that only test dict[str, str]. Although we do test that case we also test cases with more exotic key types, to allow the test to give us more confidence in this function.

I also updated the type hints in get_dict_key_from_value function itself, although they are mostly Any and basedPyright does not like the Any type by default. Although in this case I think it makes sense, dicts can have pretty much any key and value, so it makes sense that we could give this function a dict of Any and so the value we're searching for (searchval) as well could also be Any.