Improve unit testing

[whitead]

• Add C++ tests
• Add coverage analysis
• Add timing tests for assessing regression in performance

[whitead]

Feel free to split this across multiple milestones. Especially the timing tests. You can see examples of tests in hoomd-blue/test

[RainierBarrett]

We can run pytest --cov=hoomd-tf/htf hoomd-tf/htf/test-py to get code coverage reported, and for timing information, we can run benchmark_xla.py. Despite its name, it can be used to benchmark with and without use of XLA, and records execution timesteps per second in a file when it's finished.

[whitead]

Closing it for now