[FLINK-22406][coordination][tests] Stabilize ReactiveModeITCase

[zentol]

Stabilizes the ReactiveModeITCase by not checking what is actually being deployed, but just asking the JM with what parallelism it currently intends to run the job with.

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[tillrohrmann]

Thanks for creating this PR @zentol. The change itself looks good to me. As an improvement, I would suggest enriching the MiniClusterResource with a getRestClusterClient method which constructs the RestClusterClient so that we can talk to the RestServer. This will allow other tests to use a similar pattern as done here.

[tillrohrmann]

Thanks for updating the PR @zentol. LGTM. +1 for merging.