[FLINK-38908] Fix flaky ExecutionPartitionLifecycleTest

[mateczagany]

What is the purpose of the change

This fix is related to PR #26821 and sometimes these tests fail because the scheduler is being called from outside the main class.

Brief change log

• Set up executor for JM main thread and use that to interact with the Scheduler in ExecutionPartitionLifecycleTest

Verifying this change

After this change, I could no longer reproduce the failure

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, Kubernetes/Yarn, 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

[flinkbot]

CI report:

• 398754d Azure: FAILURE

Bot commands

The @flinkbot bot supports the following commands:

• @flinkbot run azure re-run the last Azure build

[mateczagany]

@flinkbot run azure