move documents to tests for simple test scenarios

[epinzur]

Does this still make sense when we are moving towards testing get_adjacent() at the adapters?

[bjchambers]

I think this still makes sense.

1. The in-memory integration tests may end up with more different sets of documents to test a wider range of cases. Thus, we may end up with more document sets.
2. In those cases, we'll likely have some document sets only used in one or two tests.

That said, we should think about how to limit the size of the test from getting too large -- we could say that for in memory tests, we create a separate test_***.py file for each document set, describing what is being tested by that, etc.?