[FLINK-10569] Remove Instance usage in FailoverRegionTest

[tisonkun]

What is the purpose of the change

Temporarily, use SimpleSlotProvider to get rid of usage of Instance. However, SimpleSlot is a legacy resource concept. A follow up should refactor SimpleSlotProvider to TestingLogicalSlotProvider or something.

Verifying this change

This change is a trivial rework / code cleanup without any test coverage.

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

• Dependencies (does it add or upgrade a dependency): (no)
• The public API, i.e., is any changed class annotated with @Public(Evolving): (no)
• The serializers: (no)
• The runtime per-record code paths (performance sensitive): (no)
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Yarn/Mesos, ZooKeeper: (no)
• The S3 file system connector:(no)

Documentation

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

cc @tillrohrmann

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[zentol]

Why can't we refactor the test to TestingLogicalSlotProvider now? What is blocking this effort?

[tisonkun]

@zentol Actually I did the refactor. The only possibly problematic change is I expose AllocatedSlot to public. Since it has a public constructor I guess it could be OK. If not so, we are going to add a testing SlotContext with similar functions as AllocatedSlot.

[zentol]

oooooki now that I see it this TestingLogicalSlotProvider business is out-of-scope for this PR. Please open a separate JIRA for that.
I will merge the first commit though.

[tisonkun]

OK I file a JIRA FLINK-11710. Is there anything I should do with thi pr? In my understanding we merge the first commit and close this pr. The second commit will be in another pr.

[zentol]

you don't have to do anything to this PR, I'll take care of it.