[Azure Pipelines] Run pytest tests (and publish results) on macOS

[foolip]

Two changes in this PR:

The decision job running in a Linux VM will decide whether or not jobs
running on other OSes start at all. Most templates are now only used
once, but most ought to be reused when running affected tests.

And run pytest tests and public results using this nifty task:
https://docs.microsoft.com/en-us/azure/devops/pipelines/tasks/test/publish-test-results

[foolip]

Pardon the rename, I hit buttons too fast. Also want to see Azure Pipelines go green for the first commit before I push the next.

[foolip]

See https://dev.azure.com/web-platform-tests/wpt/_build/results?buildId=848&view=logs for proof that the first commit works. (I intend for this PR to be rebased with separate commits.)

[foolip]

https://dev.azure.com/web-platform-tests/wpt/_build/results?buildId=850&view=logs shows the intended outcome of this. I missed the important part in the first push, it's all in the fixup :)

[foolip]

@Hexcles @lukebjerring requesting your review since it's probably a good idea for more of us to understand this setup when I add affected tests to it.

[lukebjerring]

Can confirm I read through, but any review would merely be a rubber stamp.

Is that publish results tool flexible enough for custom results ( WPT reports ) in future?

[foolip]

Is that publish results tool flexible enough for custom results ( WPT reports ) in future?

If we could convert our results into the JUnit XML format then we could see something cool for ./wpt run as well. However, I suspect we'd run into some trouble with the assumption that each test is a method and we just have a bunch of strings as our test names. Totally worth experimenting with though, there might be workarounds.

[Hexcles]

LGTM % some minor nits & questions