CNDB-17791: measure only active memtable cleanup windows in FlushFailingOnNotificationSubscriberTest

[driftx]

What is the issue

The test is flaky

What does this PR fix and why was it fixed

Track maxTimeSinceCleanup only while the memory pool actually needs cleaning, and reset the window once cleanup completes or after a successful flush.

[github-actions]

Checklist before you submit for review

• This PR adheres to the Definition of Done
• Make sure there is a PR and ticket 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 IBM copyright header instead of the Apache License one (no DataStax copyright any longer)

[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

[plpesvc-ds]

❌ Build ds-cassandra-pr-gate/PR-2418 rejected by Butler

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2 regressions found
See build details here

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Found 2 new test failures

Test	Explanation	Runs	Upstream
o.a.c.index.sai.cql.VectorCompaction100dTest.testOneToManyCompaction[version=ec enableNVQ=true] ()	NEW	🔴	0 / 30
o.a.c.index.sai.cql.VectorSiftSmallTest.testRerankKZeroOrderMatchesFullPrecisionSimilarity[version=ed enableNVQ=true enableFused=false] ()	NEW	🔴	0 / 30

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Found 3 known test failures