Fix data stream retrieval in DataStreamLifecycleServiceRuntimeSecurityIT (#125667)

This test seemed to fail because we would index another document before
the master processed the data stream rollover - which is possible
because non-master nodes expose the next cluster state a little bit
earlier than the master node.

Caused by #122852

Fixes #124978

Author: nielsbauman

muted-tests.yml

@@ -302,9 +302,6 @@ tests:
 - class: org.elasticsearch.xpack.esql.action.CrossClusterAsyncQueryStopIT
   method: testStopQueryLocal
   issue: https://github.com/elastic/elasticsearch/issues/121672
-- class: org.elasticsearch.integration.DataStreamLifecycleServiceRuntimeSecurityIT
-  method: testRolloverLifecycleAndForceMergeAuthorized
-  issue: https://github.com/elastic/elasticsearch/issues/124978
 - class: org.elasticsearch.packaging.test.BootstrapCheckTests
   method: test10Install
   issue: https://github.com/elastic/elasticsearch/issues/124957

x-pack/plugin/security/src/internalClusterTest/java/org/elasticsearch/integration/DataStreamLifecycleServiceRuntimeSecurityIT.java

@@ -97,23 +97,19 @@ public void testRolloverLifecycleAndForceMergeAuthorized() throws Exception {
         // empty lifecycle contains the default rollover
         prepareDataStreamAndIndex(dataStreamName, DataStreamLifecycle.Template.DEFAULT);
 
-        assertBusy(() -> {
-            assertNoAuthzErrors();
-            List<Index> backingIndices = getDataStreamBackingIndices(dataStreamName);
-            assertThat(backingIndices.size(), equalTo(2));
-            String backingIndex = backingIndices.get(0).getName();
-            assertThat(backingIndex, backingIndexEqualTo(dataStreamName, 1));
-            String writeIndex = backingIndices.get(1).getName();
-            assertThat(writeIndex, backingIndexEqualTo(dataStreamName, 2));
-        });
+        List<String> backingIndices = waitForDataStreamBackingIndices(dataStreamName, 2);
+        String backingIndex = backingIndices.get(0);
+        assertThat(backingIndex, backingIndexEqualTo(dataStreamName, 1));
+        String writeIndex = backingIndices.get(1);
+        assertThat(writeIndex, backingIndexEqualTo(dataStreamName, 2));
+
+        assertNoAuthzErrors();
         // Index another doc to force another rollover and trigger an attempted force-merge. The force-merge may be a noop under
         // the hood but for authz purposes this doesn't matter, it only matters that the force-merge API was called
         indexDoc(dataStreamName);
-        assertBusy(() -> {
-            assertNoAuthzErrors();
-            List<Index> backingIndices = getDataStreamBackingIndices(dataStreamName);
-            assertThat(backingIndices.size(), equalTo(3));
-        });
+
+        waitForDataStreamBackingIndices(dataStreamName, 3);
+        assertNoAuthzErrors();
     }
 
     public void testRolloverAndRetentionAuthorized() throws Exception {