The GifUtil namespace needs unit tests

Write unit tests for the functions in the GifUtil namespace.

A number of unit tests elsewhere make use of them, but we don't have full coverage, and there should be separate tests anyway for purposes of isolating problems.

I have a separate app that makes extensive use of all of GifUtil, so I'm mostly sure they're working at this point. The tests will help with maintenance over time.

UPDATE: Other than color quantization, which has a test suite.