Add testing workflow

[aktech]

This adds test workflow for tfx-bsl.

TODO:

• Create reusable workflow to extract common stuff form build and test workflow
• Remove trigger on all "*" branches
• Add xfail to failing tests
• Add upload to PyPi and twine check

[peytondmurray]

A few changes requested (thanks to the helpful comments here).

Sadly the use of unittest really highlights problems that apache-beam feels as a result of dill unconditionally trying to pickle arbitrary objects. Moving forward we should move away from unittest.TestCase methods for carrying out assertions such as self.assertEqual, as this requires pickling the unittest.TestCase itself - which fails. For now, xfailing makes sense.

[peytondmurray]

Nice! 🚀