Add Gengo API tests and skip_if infrastructure

This adds some skip_if infrastructure for skipping tests if an
expression evaluates to True.

It also adds a couple of tests that actually execute the Gengo API calls
rather than executing against a mock Gengo API.

I should have done this when I did the machine translation changes.

r?
It means these are not mocked and are running against a real API with real keys. They're LIVE and not SIMULATED.

If you have ideas on how to clarify that in the docstring, I'm game.