Work-in-progress refactor: switch gateway tests to pytest

[klauer]

Goal

• Convert unittest-based tests into pytest and refactor around built-ins such as assert
  • This is cleaned/pared down from a previous effort in a separate repository for SLAC (LCLS, PCDS) to verify new gateway deployments
  • There are new tests here: a description will be added later
• Add junit-xml output for tests (included as an artifact)
• Add test result summary (included as an artifact which seems to render in the GH actions page as well)

Work-in-progress

Remaining to do:

• Describe the newly-added tests that weren't just an update/conversion
• Look into recent epics-extensions/ca-gateway changes that happened after these were converted to pytest (and forked into the other repository)
• Open issues for expected-but-not-yet-reported test failures (mostly related to caPutLog handling)
• Investigate differing results from what we saw previously
• See if there are any better test summarizing tools
• Update 3.14/3.15 to test put logging (or disable those tests entirely)

[ralphlange]

I hope you closed it to be replaced by a rebased equivalent?!

[klauer]

Ah, was cleaning up my old PRs and closed this by mistake - would you mind approving the CI to run?

[klauer]

@ZLLentz maybe this is something you could take over at some point? (cc @tangkong)