TEST: Ensure merge triggered in _source retention test (#33487)

We invoke force merge twice in the test to verify that recovery sources
are pruned when the global checkpoint advanced. However, if the global
checkpoint equals to the local checkpoint in the first force-merge, the
second force-merge will be a noop because all deleted docs are expunged
in the first merge already. We need to flush a new segment to make merge
happen so we can verify that all recovery sources are pruned.

Author: dnhatn

server/src/test/java/org/elasticsearch/index/engine/InternalEngineTests.java

@@ -1522,6 +1522,16 @@ public void testForceMergeWithSoftDeletesRetentionAndRecoverySource() throws Exc
             settings.put(IndexSettings.INDEX_SOFT_DELETES_RETENTION_OPERATIONS_SETTING.getKey(), 0);
             indexSettings.updateIndexMetaData(IndexMetaData.builder(defaultSettings.getIndexMetaData()).settings(settings).build());
             engine.onSettingsChanged();
+            // If the global checkpoint equals to the local checkpoint, the next force-merge will be a noop
+            // because all deleted documents are expunged in the previous force-merge already. We need to flush
+            // a new segment to make merge happen so that we can verify that all _recovery_source are pruned.
+            if (globalCheckpoint.get() == engine.getLocalCheckpoint() && liveDocs.isEmpty() == false) {
+                String deleteId = randomFrom(liveDocs);
+                engine.delete(new Engine.Delete("test", deleteId, newUid(deleteId), primaryTerm.get()));
+                liveDocsWithSource.remove(deleteId);
+                liveDocs.remove(deleteId);
+                engine.flush();
+            }
             globalCheckpoint.set(engine.getLocalCheckpoint());
             engine.syncTranslog();
             engine.forceMerge(true, 1, false, false, false);