test to detect (platform specific) plugged-in scheduler

[yannicks1]

As requested by the reviewers for this PR, here is a unit test to validate the functionality of the platform specific pluggable scheduler.

working principle:
Via check_and_update_config() of a DummyPlatform we set the scheduler_cls of the parallel configs to point to a (platform specific) DummyScheduler. The DummyScheduler raises a ValueError which is caught in the pytest for validation.

[github-actions]

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