CNDB-15519 test BM25 after compaction too

[k-rus]

What is the issue

Fixes https://github.com/riptano/cndb/issues/15519
Compaction might write SAI differently than flush, thus it's good to have tests running after compaction.

What does this PR fix and why was it fixed

Changes to use runThenFlushThenCompact instead of beforeAndAfterFlush in BM25 test. Another test already tests after compaction in addition to memtable and after flush.

[github-actions]

Checklist before you submit for review

• This PR adheres to the Definition of Done
• 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-2026 approved by Butler

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Approved by Butler
See build details here