Automatically generate fixture names and strengthen test cases

[damemi]

Discussed in #613, it would be good if our output fixture names were automatically generated based on the test name. This would help prevent duplicate fixture overwriting, but it would also more clearly identify the test cases each fixture is meant to represent.

One benefit we have right now though is that fixtures are pretty tightly organized, eg:

• input_fixture.json
• input_fixture_expected.json
• input_fixture_alternate_expected.json

Especially in the third case, where 2 tests use the same input fixture (which is fine), having the outputs all colocated with the input is convenient. We probably won't be able to generate the input fixture name based on the test name (especially because multiple tests can use the same input), so we'll have to put some thought into what this actually looks like.

[aabmass]

Triaged, this would be nice but not high priority