CNDB-17830: Fix flaky PrefetchingRebufferer non-sequential access test

[driftx]

Wait for asynchronous cleanup to complete before asserting request counts.

[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-2423 rejected by Butler

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3 regressions found
See build details here

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Found 3 new test failures

Test	Explanation	Runs	Upstream
o.a.c.index.sai.cql.VectorCompaction100dTest.testZeroOrOneToManyCompaction[version=ec enableNVQ=false] ()	NEW	🔴	0 / 30
o.a.c.index.sai.cql.VectorSiftSmallTest.testMultiSegmentBuild[version=fa enableNVQ=false enableFused=false] ()	NEW	🔴	0 / 30
o.a.c.index.sai.disk.RowAwareWidePrimaryKeyMapTest.testCeiling[version=ca] (compression)	REGRESSION	🔴	0 / 30

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Found 3 known test failures