[FLINK-25925][tests] Require slot pool to only accept the same set of slots as before

[tillrohrmann]

Instead of also accepting excess slots, we only need to require the same set of slots
as before. This commit changes the JobMasterTest.testJobMasterAcceptsExcessSlotsWhenJobIsRestarting
accordingly.

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[tillrohrmann]

Thanks for the review @zentol. Merging this PR now.