Flaky test: Add condition to ensure data correctness on primary promotion (#8431) (#8495)

(cherry picked from commit bcaf494)

Signed-off-by: Ankit Kala <[email protected]>
Signed-off-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>
Co-authored-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>

Author: opensearch-trigger-bot[bot]

server/src/internalClusterTest/java/org/opensearch/indices/replication/SegmentReplicationIT.java

@@ -732,6 +732,7 @@ public void testDropPrimaryDuringReplication() throws Exception {
             // start another replica.
             dataNodes.add(internalCluster().startDataOnlyNode());
             ensureGreen(INDEX_NAME);
+            waitForSearchableDocs(initialDocCount, dataNodes);
 
             // index another doc and refresh - without this the new replica won't catch up.
             String docId = String.valueOf(initialDocCount + 1);