Speed up test suite execution

[jphalip]

The overall test suite is currently a bit slow to run. Here are a few things to look into to speed things up:

• Convert some integration tests into unit tests. We might not need to exercise the full stack for so many tests. Instead some integration tests could be modified to directly test specific classes or methods.
• Reduce test matrix for some tests. We currently use CartesianTest to run a given test under multiple combination of configurations: execution engine (tez/mr), read format (avro/arrow), and write method (direct/indirect). This is to make sure we test all possible code paths. But it might be redundant to do that for all tests. We could reduce the test matrix for some tests after inspecting the test coverage and making sure that the various code paths are well covered.