Add python tests

[david4096]

Now that we are able to easily regenerate the python bindings for the protobuf, we could provide testing of the schemas directly. This allows us to specify validation of fields beyond what protobuf types. For example, we can construct tests showing valid and invalid Variant representations and enforcing variant.start > variant.end.

[david4096]

We can close this based on https://github.com/ga4gh/schemas/blob/master/tests/test_imports.py