Migrate IndicesExistsIT to Zen2 (#37526)

This test was actually passing, for the wrong reason: it asserts a
`MasterNotDiscoveredException` is thrown, expecting this to be due to a failure
to perform state recovery, but in fact it's thrown because the node is not
correctly bootstrapped.

server/src/test/java/org/elasticsearch/action/admin/indices/exists/IndicesExistsIT.java

@@ -19,6 +19,7 @@
 
 package org.elasticsearch.action.admin.indices.exists;
 
+import org.elasticsearch.cluster.coordination.ClusterBootstrapService;
 import org.elasticsearch.common.settings.Settings;
 import org.elasticsearch.common.unit.TimeValue;
 import org.elasticsearch.discovery.MasterNotDiscoveredException;
@@ -29,17 +30,29 @@ import org.elasticsearch.test.ESIntegTestCase.ClusterScope;
 import org.elasticsearch.test.InternalTestCluster;
 
 import java.io.IOException;
+import java.util.List;
+import java.util.stream.Collectors;
 
+import static org.elasticsearch.node.Node.NODE_MASTER_SETTING;
+import static org.elasticsearch.node.Node.NODE_NAME_SETTING;
 import static org.elasticsearch.test.hamcrest.ElasticsearchAssertions.assertThrows;
 
 @ClusterScope(scope = ESIntegTestCase.Scope.TEST, numDataNodes = 0, numClientNodes = 0, transportClientRatio = 0.0,
     autoMinMasterNodes = false)
 public class IndicesExistsIT extends ESIntegTestCase {
 
+    @Override
+    protected List<Settings> addExtraClusterBootstrapSettings(List<Settings> allNodesSettings) {
+        final List<String> masterNodeNames
+            = allNodesSettings.stream().filter(NODE_MASTER_SETTING::get).map(NODE_NAME_SETTING::get).collect(Collectors.toList());
+        return allNodesSettings.stream().map(s -> Settings.builder().put(s)
+            .putList(ClusterBootstrapService.INITIAL_MASTER_NODES_SETTING.getKey(), masterNodeNames).build()).collect(Collectors.toList());
+    }
+
     public void testIndexExistsWithBlocksInPlace() throws IOException {
         Settings settings = Settings.builder()
             .put(GatewayService.RECOVER_AFTER_NODES_SETTING.getKey(), 99)
-            .put(ElectMasterService.DISCOVERY_ZEN_MINIMUM_MASTER_NODES_SETTING.getKey(), 1).build();
+            .put(ElectMasterService.DISCOVERY_ZEN_MINIMUM_MASTER_NODES_SETTING.getKey(), Integer.MAX_VALUE).build();
         String node = internalCluster().startNode(settings);
 
         assertThrows(client(node).admin().indices().prepareExists("test").setMasterNodeTimeout(TimeValue.timeValueSeconds(0)),