Soup up test data handling

[GretaCB]

Per #5 (comment), we can improve tests by updating them to better handle test data and ensure proper comparisons/assertions:

• file based fixtures for input data
• an adjacent file of gzip coded data
• then compare them byte for byte
• optional: add an UPDATE environment variable to be able to put the tests into a mode where they are updateable

This way we can ensure the output is exactly as we expect. And we would pick up differences, say, between our results and another implementation like #7.

cc @springmeyer

[springmeyer]

👍 Good plan.

[springmeyer]

Assigning to Blue Sky milestone because we've got pretty good unit tests so far. This would be nice but does not seem critical for a first release.