CNDB-15580: CNDB-15519 test BM25 after compaction too (#2026)

Compaction might write SAI differently than flush, thus it's good to
have tests running after compaction.

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

Author: k-rus

test/unit/org/apache/cassandra/index/sai/cql/BM25Test.java

@@ -793,7 +793,7 @@ public void testCollections() throws Throwable
         analyzeDataset("climate");
         analyzeDataset("health");
 
-        beforeAndAfterFlush(
+        runThenFlushThenCompact(
         () -> {
             executeQuery(CLIMATE_QUERY_RESULTS, "SELECT * FROM %s  ORDER BY body BM25 OF ? LIMIT 10",
                          "climate");