Flaky test: Add condition to ensure data correctness on primary promo…

…tion (#8431)

Signed-off-by: Ankit Kala <ankikala@amazon.com>

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

@@ -733,6 +733,7 @@ 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);