Split up tests?

Currently a large share of our tests are in test_dataset.py and test_dataarray.py — each of which are around 7k lines.

There's a case for splitting these up:

• Many of the tests are somewhat duplicated between the files (and test_variable.py in some cases) — i.e. we're running the same test over a Dataset & DataArray, but putting them far away from each other in separate files. Should we instead have them split by "function"; e.g. test_rolling.py for all rolling tests?
• My editor takes 5-20 seconds to run the linter and save the file. This is a very narrow complaint.
• Now that we're all onto pytest, there's no need to have them in the same class.

If we do this, we could start on the margin — new tests around some specific functionality — e.g. join / rolling / reindex / stack (just a few from browsing through) — could go into a new respective test_{}.py file. Rather than some big copy and paste commit.

Start by reviewing test_dataset.py, test_dataarray.py, and test_variable.py, focusing on duplicated tests for join, rolling, reindex, or stack. A possible first step is placing new tests for one functionality in a respective test_{}.py file; completion would mean the tests are organized without a broad copy-and-paste migration.