Fix testRolloverForFreshInstalledIndexTemplate (elastic#116450)

Fix IndexTemplateRegistryTests.testRolloverForFreshInstalledIndexTemplate
by using assertBusy, now that we're expecting a rollover.

x-pack/plugin/core/src/test/java/org/elasticsearch/xpack/core/template/IndexTemplateRegistryTests.java

@@ -475,8 +475,7 @@ public void testRolloverForFreshInstalledIndexTemplate() throws Exception {
         assertBusy(() -> assertThat(putIndexTemplateCounter.get(), equalTo(1)));
         // rollover should be triggered even for the first installation, since the template
         // may now take precedence over a data stream's existing index template
-        Thread.sleep(100L);
-        assertThat(rolloverCounter.get(), equalTo(2));
+        assertBusy(() -> assertThat(rolloverCounter.get(), equalTo(2)));
     }
 
     public void testThatTemplatesAreNotUpgradedWhenNotNeeded() throws Exception {