Clean up tests

[drwelby]

Tests are a mess.

• Get rid of out-of-date instructions in each test
• Write a single readme for testing
• Update all tests to latest GBDX_MOCK access pattern
• Rename the pattern so it makes sense
• Standardize the VCRpy defaults so we don't have to pass the same options every time.

[drwelby]

Initial cleanup pattern in #805

• standardize VCRpy options
• get rid of weird FORCE matcher, why?!?!?!?
• made mockable interface make more sense

Need to add:

• standardize catIDs to use images that are on the "Never Delete" list.

[drwelby]

On the whole, these tests need to shift to more mocking and less VCR recording.

[drwelby]

Tests that all require initializing the same image (and no other calls) should use one cassette.