[FLINK-39773][tests] Use random ports in ClusterEntryPointTest

[snuyanzin]

What is the purpose of the change

This test is failing in java25 since the ports are going to be free in async mode, in java 25 tests might started before ports are freed from previous test.
To make it more reliable: the PR makes using random ports
another way: making sure ports are freed before next test is started will require more changes

Brief change log

ClusterEntryPointTest

Verifying this change

the change is test itself
my CI with java25 where this test fails https://dev.azure.com/snuyanzin/flink/_build/results?buildId=4810&view=logs&j=7f652c99-c3cd-5aee-11e2-f8e88140dbea&t=d3173862-60e8-57db-ccd6-465075b97bea&l=8802

same ci with this fix (now the next test failed which will be fixed separately)
https://dev.azure.com/snuyanzin/flink/_build/results?buildId=4836&view=logs&j=9dc1b5dc-bcfa-5f83-eaa7-0cb181ddc267&t=e72e9469-7bd1-5a7c-cfdb-1df1c1c7d1dc&l=10544

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 )

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Was generative AI tooling used to co-author this PR?

• Yes (please specify the tool below)

[flinkbot]

CI report:

• e061189 Azure: SUCCESS

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[snuyanzin]

@gaborgsomogyi I noticed you were contributed here a couple of years ago
do you mind to take a look?

[gaborgsomogyi]

Assuming green with all JVM versions.

[snuyanzin]

thanks for taking a look
yep, for other javas also passing