Speed-up CI by reorganizing tests (#22247)

* unit tests: mark slow tests as "maybeslow"

This commit also removes the "network" marker and
marks every "network" test as "maybeslow". Tests
marked as db are maintained, but they're not slow
anymore.

* GA: require style tests to pass before running unit-tests

* GA: make MacOS unit tests fail fast

* GA: move all unit tests into the same workflow, run style tests as a prerequisite

All the unit tests have been moved into the same workflow so that a single
run of the dorny/paths-filter action can be used to ask for coverage based
on the files that have been changed in a PR. The basic idea is that for PRs
that introduce only changes to packages coverage is not necessary, this
resulting in a faster execution of the tests.

Also, for package only PRs slow unit tests are skipped.

Finally, MacOS and linux unit tests are now conditional on style tests passing
meaning that e.g. we won't waste a MacOS worker if we know that the PR has
flake8 issues.

* Addressed review comments

* Skipping slow tests on MacOS for package only recipes

* QA: make tests on changes correct before merging

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