setting the response before latch countdown so that thread waiting do…

…es not encounter null (opensearch-project#13999)

Signed-off-by: Rajiv Kumar Vaidyanathan <rajivkv@amazon.com>
Signed-off-by: kkewwei <kkewwei@163.com>

server/src/internalClusterTest/java/org/opensearch/ratelimitting/admissioncontrol/AdmissionForClusterManagerIT.java

@@ -170,8 +170,8 @@ public void testAdmissionControlResponseStatus() throws Exception {
 
             @Override
             public void sendResponse(RestResponse response) {
-                waitForResponse.countDown();
                 aliasResponse.set(response);
+                waitForResponse.countDown();
             }
         };