[FLINK-11594][tests] Remove TaskManagerRegistrationTest

[GJL]

What is the purpose of the change

This removes TaskManagerRegistrationTest. Tests were mostly covered already.

Test in TaskManagerRegistrationTest	Covered by
testSimpleRegistration	TaskExecutorTest#testImmediatelyRegistersIfLeaderIsKnown, TaskExecutorTest#testOfferSlotToJobMasterAfterTimeout, TaskExecutorTest#testHeartbeatTimeoutWithJobManager, ResourceManagerTaskExecutorTest#testRegisterTaskExecutor, TaskExecutorToResourceManagerConnectionTest
testDelayedRegistration	RetryingRegistrationTest#testRetryConnectOnFailure
testShutdownAfterRegistrationDurationExpired	TaskExecutor#testMaximumRegistrationDuration, TaskManagerRunnerTest#testShouldShutdownIfRegistrationWithJobManagerFails, TaskManagerRunnerTest#testShouldShutdownOnFatalError
testTaskManagerResumesConnectAfterRefusedRegistration	RetryingRegistrationTest#testDecline
testTaskManagerNoExcessiveRegistrationMessages	Backoff time is tested in several tests in RetryingRegistrationTest
testTaskManagerResumesConnectAfterJobManagerFailure	TaskExecutorTest#testHeartbeatTimeoutWithJobManager, TaskExecutorTest#testHeartbeatTimeoutWithResourceManager, ZooKeeperLeaderElectionITCase#testJobExecutionOnClusterWithLeaderChange
testCheckForValidRegistrationSessionIDs	TM does not have the logic under test anymore. It is enough to test that the TM connects to the leading JM (e.g., TaskExecutorTest#testJobLeaderDetection)

Brief change log

• See commits

Verifying this change

This change is already covered by existing tests, such as TaskExecutorTest, RetryingRegistrationTest.

This change added tests and can be verified as follows:

• Added unit test TaskExecutorToResourceManagerConnection to test that the correct arguments are passed in the TM -> RM registration.

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): (yes / no)
• The public API, i.e., is any changed class annotated with @Public(Evolving): (yes / no)
• The serializers: (yes / no / don't know)
• The runtime per-record code paths (performance sensitive): (yes / no / don't know)
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Yarn/Mesos, ZooKeeper: (yes / no / don't know)
• The S3 file system connector: (yes / no / don't know)

Documentation

• Does this pull request introduce a new feature? (yes / no)
• If yes, how is the feature documented? (not applicable / docs / JavaDocs / not documented)

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[tillrohrmann]

Thanks for opening this PR @GJL. LGTM. The only thing missing is the actual removal of the TaskManagerRegistrationTest. After adding this, +1 for merging.

@flinkbot approve all.

[tillrohrmann]

Good refactorings!

[GJL]

Thanks for the review. I added a commit to remove TaskManagerRegistrationTest, and rebased to latest master. Merging as soon as build is green.