Azdo CI to run python tests

[DonJayamanne]

Add a job that will run pytest (I think this on), unittest, or whichever test framework we decide to use, on the Python scripts that we manage and that ship with our extension.

Test for completion

• Run as part of the PR-validation
• Run as part of the Nightly CI
• Succeed across all OS, all Python version phases - no skipping.
• Produce junit-xml style output that can be uploaded to AzDO (so that the results show up in the Test tab of each build).
• Do not significantly add to the time it takes to run a build

Tips

We should be able to simply add a new phase to the build/ci/templates/test-phase-job.yaml file that does the python tests.

Using pytest might cause issues with running on Python 2 phases. We might have to consider using unittest here, or perhaps an older version of pytest.