[FLINK-12015] Fix TaskManagerRunnerTest instability

[aljoscha]

Before, the was a race condition between the termination future in
TaskManagerRunner completing and the asynchronous shutdown part here:
https://github.com/apache/flink/blob/70107c4647ecac3df9b2b8c7920e7cb99ad550f1/flink-runtime/src/main/java/org/apache/flink/runtime/taskexecutor/TaskManagerRunner.java#L258

The test would go out of the block that was waiting on the future but
the shutdown code that is executed after the future completes is
executed asynchronously, so is not guaranteed to have run at that point.

This also refactors the code a bit to make it more obvious what is
happening and removes the SecurityManagerContext because it was
obscuring the problem.

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[tillrohrmann]

Thanks for hardening the test @aljoscha. This is a good fix. I would try to avoid busy looping by letting the SystemExitTrackingSecurityManager return a future which is completed once System.exit is called.

[tillrohrmann]

Instead of busy looping, I would prefer to change the SystemExitTrackingSecurityManager to return a CompletableFuture<Integer> which is completed with the exit code with which System.exit(exitCode) is called.

[aljoscha]

@tillrohrmann I updated the PR. Btw, this test only ensures that we have at least one call to System.exit(). This is also true for the current version on master but this fact is hidden.

[tillrohrmann]

Changes look good to me. Thanks for the hardening of this test @aljoscha. +1 for merging.

[aljoscha]

Thanks! I'll merge