[FLINK-14454] [test-stability] Usages of SavepointSerializers.setFailWhenLegacyStateDetected are never reverted

[Jiayi-Liao]

What is the purpose of the change

Re-enable FAIL_WHEN_LEGACY_STATE_DETECTED after all state migration tests.

Brief change log

Adding the revert logic in a @afterclass method.

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Last check on commit 605ff45 (Wed Dec 04 14:49:25 UTC 2019)

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[StephanEwen]

Thank you for the patch. Merging this...