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?
Hmm. That makes sense, and I can't think of a better wording to imply that concisely. I think it's fine as is.