[FLINK-21954][tests] Harden JobMasterQueryableStateTest, JobMasterTest.testRestoringFromSavepoint and .testRequestNextInputSplitWithGlobalFailover

[tillrohrmann]

This commit fixes the test instabilities JobMasterQueryableStateTest.*, JobMasterTest.testRestoringFromSavepoint and JobMasterTest.testRequestNextInputSplitWithGlobalFailover which were introduced because of FLINK-21602 which makes the ExecutionGraph creation asynchronous. Due to this change, we now have to wait until a task is submitted in order to ensure that the ExecutionGraph has been created.

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[rmetzger]

Thanks a lot for fixing the test instability!

[tillrohrmann]

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