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?
What do you mean by "LIVE" here? It makes me think that this is related to Django's LiveServerTestCase, but I don't think that's true. I like that you emphasized the gravity of these tests.