Add unit test reports to L0_json, L0_metrics, L0_response_cache, L0_backend_python

[rmccorm4]

• Generates junit test reports from various test and subtests. This is not an exhaustive list of possible CI tests, just an initial set.
  • Had to remove use of tu.TestResultCollector as it interfered with pytest usage. Wasn't able to find a quick reason/solution as to why - it involved the use of super() and certain expected object types. Reverting the classes to inheriting from unittest.TestCase directly fixed the issue.
  • Similarly, removed the use of check_test_results since the tu.TestResultCollector was no longer generating the text file with pass/fail counts. For the short term, I think the numbers shown in gitlab from the reports can replace this for now. Longer term, I think we can modify check_test_results to understand the .xml reports as well, rather than our custom .json.

Example report generated on gitlab side:

[krishung5]

Lgtm. Great improvements!