switch testing framework for pytest to abseil

[sina chavoshi]

https://abseil.io/docs/python/guides/testing, this will allow us to benefit from some of the test automated infrastructure namely be able to share nicely formatted test results back on GH for integration tests.

[Makoto Uchida]

One of the benefit of abseil testing framework is ability to parameterize test cases. We could potentially remove lots of repetition.

For other parts, I believe it's almost (completely?) upward compatible with unittest, so it should be mostly drop-in replacement.