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.
[bug 1026131] Add Gengo human translations bookkeeping
[bug 1026131] Rework translations system logging code
[bug 1027293] Add audit records to response view

They look ugly, but that's fine for now.