Add separate CI job for slow tests

[carmocca]

What does this PR do?

Just 1 parametrization for the moment, we can always add more later if necessary.

If I forgot to mark a specific test as slow=True, feel free to commit or comment.

Fixes #9086

Does your PR introduce any breaking changes? If yes, please list them.

None

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cc @carmocca @akihironitta @Borda

[tchaton]

LGTM !

[awaelchli]

Not sure if it was discussed, but was it intentional to skip slow tests by default? This means locally all tests marked as slow don't run. Since locally we usually run only a subset of tests anyway, "slowness" doesn't play such a big role. And for debugging purposes, it may be good to have them run always. May I suggest to invert the current behavior?

[carmocca]

Skipping them by default is the point of adding Slow CI, otherwise, they get to run on all jobs and we get no speedup.

[awaelchli]

You would make the default to run the slow tests and set

PL_RUN_SLOW_TESTS=0 in the regular CI and
PL_RUN_SLOW_TESTS=1 in the dedicated one.

Currently only the slow ci configuration sets the env variable.

[carmocca]

We would need 3 values:

1. Run only slow tests (current PL_RUN_SLOW_TESTS=1)
2. Run only non-slow tests. Current default. (current PL_RUN_SLOW_TESTS=0)
3. Run all tests. Suggested default. How do you propose we set it?

I personally prefer the current default because I want to run the quick tests much more often than all tests, and the slowness of the slow tests (as a Mac user) makes them too painful.

[Borda]

just noticed that here we skip run on Windows and all minimal requirements?

[carmocca]

Yes, I wanted to keep this light at first as we don't run many tests at the moment. These can be added later