[Test] Reduce concurrency when testing creation of security index (elastic#75293)

The internal cluster could get overwhelmed by many nodes and large
number of concurrent putUser request. It can sometimes fail with
confusing messages when JVM is under pressure. This PR reduces the
concurrency so it has a better chance to succeed.

The test also no longer relies on the user being "created" instead of "updated".
Since they do not make a difference for the purpose of this test.

Author: ywangd

x-pack/plugin/security/src/internalClusterTest/java/org/elasticsearch/xpack/security/support/SecurityIndexManagerIntegTests.java

@@ -28,9 +28,10 @@ public class SecurityIndexManagerIntegTests extends SecurityIntegTestCase {
     public void testConcurrentOperationsTryingToCreateSecurityIndexAndAlias() throws Exception {
         assertSecurityIndexActive();
         final int processors = Runtime.getRuntime().availableProcessors();
-        final int numThreads = scaledRandomIntBetween((processors + 1) / 2, 4 * processors);
-        final int maxNumRequests = 100 / numThreads; // bound to a maximum of 100 requests
+        final int numThreads = Math.min(50, scaledRandomIntBetween((processors + 1) / 2, 4 * processors));  // up to 50 threads
+        final int maxNumRequests = 50 / numThreads; // bound to a maximum of 50 requests
         final int numRequests = scaledRandomIntBetween(Math.min(4, maxNumRequests), maxNumRequests);
+        logger.info("creating users with [{}] threads, each sending [{}] requests", numThreads, numRequests);
 
         final List<ActionFuture<PutUserResponse>> futures = new CopyOnWriteArrayList<>();
         final List<Exception> exceptions = new CopyOnWriteArrayList<>();
@@ -75,7 +76,10 @@ protected void doRun() throws Exception {
         assertThat(exceptions, Matchers.empty());
         assertEquals(futures.size(), numRequests * numThreads);
         for (ActionFuture<PutUserResponse> future : futures) {
-            assertTrue(future.actionGet().created());
+            // In rare cases, the user could be updated instead of created. For the purpose of
+            // this test, either created or updated is sufficient to prove that the security
+            // index is created. So we don't need to assert the value.
+            future.actionGet().created();
         }
     }