[FLINK-38441][runtime] Fix ExecutionGraphCoLocationRestartTest failures

[mateczagany]

What is the purpose of the change

Fix ExecutionGraphCoLocationRestartTest intermittent failures that happen because of tasks being executed outside of the main thread. This is related FLINK-38114, PR #26821

Brief change log

• Update test so that all ExecutionGraph operations happen from the main thread

Verifying this change

Could no longer reproduce the same failure after this fix.

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:

• a518b9d Azure: FAILURE

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[mateczagany]

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