Fix DiscoveryDisruptionIT.testElectMasterWithLatestVersion

The clusterAdmin() call at the end of the test uses a random node,
which may not have been part of the current election, and may still
have an old cluster state, prior to the creation of the index in the
test.  The fix uses awaitClusterState() to wait for a non-null master
and saves the state to check for the test index.

Resolves: #134748

Author: JeremyDahlgren

muted-tests.yml

@@ -516,9 +516,6 @@ tests:
 - class: org.elasticsearch.xpack.esql.expression.function.scalar.score.DecayTests
   method: "testEvaluateBlockWithNulls {TestCase=<integer>, <integer>, <integer>, <_source> #2}"
   issue: https://github.com/elastic/elasticsearch/issues/134679
-- class: org.elasticsearch.discovery.DiscoveryDisruptionIT
-  method: testElectMasterWithLatestVersion
-  issue: https://github.com/elastic/elasticsearch/issues/134748
 - class: org.elasticsearch.xpack.esql.ccq.MultiClusterSpecIT
   method: test {csv-spec:fork.ForkBeforeStatsByWithWhere}
   issue: https://github.com/elastic/elasticsearch/issues/134817

server/src/internalClusterTest/java/org/elasticsearch/discovery/DiscoveryDisruptionIT.java

@@ -30,6 +30,7 @@
 import java.util.Set;
 import java.util.concurrent.CountDownLatch;
 import java.util.concurrent.CyclicBarrier;
+import java.util.concurrent.atomic.AtomicReference;
 
 /**
  * Tests for discovery during disruptions.
@@ -146,8 +147,14 @@ public void testElectMasterWithLatestVersion() throws Exception {
 
         isolateAllNodes.stopDisrupting();
 
-        awaitMasterNode();
-        final ClusterState state = clusterAdmin().prepareState(TEST_REQUEST_TIMEOUT).get().getState();
+        final var stateRef = new AtomicReference<ClusterState>();
+        awaitClusterState(state -> {
+            stateRef.set(state);
+            return state.nodes().getMasterNode() != null;
+        });
+
+        final var state = stateRef.get();
+        assertNotNull(state);
         if (state.metadata().getProject().hasIndex("test") == false) {
             fail("index 'test' was lost. current cluster state: " + state);
         }