CNDB-13238: Fix flaky CounterLockManagerTest interruption tests that don't properly wait for a finally block to complete (#1820)

[driftx]

What is the issue

The final assert in this test checks that the locks have been cleaned up. These need to be released twice; once by the main thread, and once in the finally block of a thread started by the test. The test waits for a CompletableFuture to be set by this thread, but it does not wait for the finally block to finish.

What does this PR fix and why was it fixed

Join on the other thread in the test, which ensures the finally block completes before the last assertion that checks all refs to the locks have been released.

[driftx]

Clean merge.

[github-actions]

Checklist before you submit for review

• Make sure there is a PR in the CNDB project updating the Converged Cassandra version
• Use NoSpamLogger for log lines that may appear frequently in the logs
• Verify test results on Butler
• Test coverage for new/modified code is > 80%
• Proper code formatting
• Proper title for each commit staring with the project-issue number, like CNDB-1234
• Each commit has a meaningful description
• Each commit is not very long and contains related changes
• Renames, moves and reformatting are in distinct commits
• All new files should contain the DataStax copyright header instead of the Apache License one

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[cassci-bot]

❌ Build ds-cassandra-pr-gate/PR-1879 rejected by Butler

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31 new test failure(s) in 1 builds
See build details here

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Found 31 new test failures

Showing only first 15 new test failures

Test	Explanation	Branch history	Upstream history
...lidation.operations.AlterTest-compression_jdk11	regression	🔴
...nQueryShouldNotTimeoutWhenItExceedesReadTimeout	regression	🔴
...nglePageReadIsFastButAggregationExceedesTimeout	regression	🔴
...adCommitLogAndSSTablesWithDroppedColumnTestCC40	regression	🔴
...adCommitLogAndSSTablesWithDroppedColumnTestCC50	regression	🔴
...oadCommitLogAndSSTablesWithDroppedColumnTestDSE	regression	🔴
...thRestartTest.testReadingValuesOfDroppedColumns	regression	🔴
o.a.c.d.t.s.f.FeaturesVersionSupportDBTest.testANN	regression	🔴
o.a.c.d.t.s.f.FeaturesVersionSupportDCTest.testANN	regression	🔴
o.a.c.d.t.s.f.FeaturesVersionSupportEBTest.testANN	regression	🔴
...c.FeaturesVersionSupportTest.testANNSupport[eb]	regression	🔴
....FeaturesVersionSupportTest.testGeoDistance[aa]	regression	🔴
....FeaturesVersionSupportTest.testGeoDistance[ba]	regression	🔴
...cySSTableTest.testVerifyOldDroppedTupleSSTables	regression	🔴
o.a.c.t.SSTablePartitionsTest.testDirectory	regression	🔴

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No known test failures found