Consider reducing/moving Azdo jobs from PR to nightly CI

[DonJayamanne]

This will greatly speed up Azdo CI.
Waiting for CI to pass takes a while when there are number of PRs.
This will only get worse over time, with more engineers working on PRs in parallel.

E.g.

• We don't need to run on both Linux & macOS, running just one of them should suffice.
  • This brings it down from 15 to ~8 jobs
• Remove pip install test requirements from Prebuild job
  • Save another 29s
• Move Unit tests from prebuild job
  • Save around 2m
• Leave only linux debugger tests
  • Remove windows debugger tests
  • Save another job

Note, this applies only to PRs

[d3r3kk]

I think speeding up PR validation step is worthwhile as well, and if the rest of the team agrees too, I think we can certainly use Linux steps in lieu of macOS for PR validation.

Note that we are currently calling out nightly failures in our daily standup in order to ensure we don't miss anything. We are actively looking for better ways to make the nightly CI build more discoverable and are open to ideas (we cannot put the badge at the top of our README.md, yet).

The nightly CI will still run macOS tests in full along with all other tests.

I would add the following detail:

• Move Unit tests from prebuild job
  • Move them into the same job that runs testSingleWorkspace
  • This adds 2m to the testSingleWorkspace job that runs in parallel (contrasting the prebuild job which blocks all other phases until it completes).
  • This improves our test coverage on both Linux, Windows, (and macOS for nightlies).

[d3r3kk]

This is complete as of #4775. It is fairly trivial to add/remove jobs from the matrix now. #4775 left the minimum set of builds working per PR to get us up and running, add test runs back to the matrix to improve coverage without adding a lot of time to the build.