Fix flaky test. (#8441)

Signed-off-by: Rishikesh1159 <rishireddy1159@gmail.com>

Author: Rishikesh1159

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

@@ -435,6 +435,7 @@ public void testNodeDropWithOngoingReplication() throws Exception {
         refresh(INDEX_NAME);
         blockFileCopy.countDown();
         internalCluster().stopRandomNode(InternalTestCluster.nameFilter(primaryNode));
+        ensureYellow(INDEX_NAME);
         assertBusy(() -> { assertDocCounts(docCount, replicaNode); });
         state = client().admin().cluster().prepareState().execute().actionGet().getState();
         // replica now promoted as primary should have same allocation id