Enable silent FollowersCheckerTest (#38851)

One of the test methods wasn't run because it was private. Making this method
public and fixing some issues around mocking the threadpool that otherwise would
lead to an NPE.

Author: Christoph Büscher

server/src/test/java/org/elasticsearch/cluster/coordination/FollowersCheckerTests.java

@@ -33,7 +33,6 @@
 import org.elasticsearch.test.EqualsHashCodeTestUtils.CopyFunction;
 import org.elasticsearch.test.transport.CapturingTransport;
 import org.elasticsearch.test.transport.MockTransport;
-import org.elasticsearch.threadpool.ThreadPool;
 import org.elasticsearch.threadpool.ThreadPool.Names;
 import org.elasticsearch.transport.ConnectTransportException;
 import org.elasticsearch.transport.TransportException;
@@ -73,7 +72,6 @@
 import static org.hamcrest.Matchers.not;
 import static org.hamcrest.Matchers.nullValue;
 import static org.hamcrest.core.IsInstanceOf.instanceOf;
-import static org.mockito.Mockito.mock;
 
 public class FollowersCheckerTests extends ESTestCase {
 
@@ -548,14 +546,16 @@ public String executor() {
         }
     }
 
-    private void testPreferMasterNodes() {
+   public void testPreferMasterNodes() {
         List<DiscoveryNode> nodes = randomNodes(10);
         DiscoveryNodes.Builder discoNodesBuilder = DiscoveryNodes.builder();
         nodes.forEach(dn -> discoNodesBuilder.add(dn));
         DiscoveryNodes discoveryNodes = discoNodesBuilder.localNodeId(nodes.get(0).getId()).build();
         CapturingTransport capturingTransport = new CapturingTransport();
-        TransportService transportService = capturingTransport.createTransportService(Settings.EMPTY, mock(ThreadPool.class),
-            TransportService.NOOP_TRANSPORT_INTERCEPTOR, x -> nodes.get(0), null, emptySet());
+        final Settings settings = Settings.builder().put(NODE_NAME_SETTING.getKey(), nodes.get(0).getName()).build();
+        final DeterministicTaskQueue deterministicTaskQueue = new DeterministicTaskQueue(settings, random());
+        TransportService transportService = capturingTransport.createTransportService(Settings.EMPTY,
+                deterministicTaskQueue.getThreadPool(), TransportService.NOOP_TRANSPORT_INTERCEPTOR, x -> nodes.get(0), null, emptySet());
         final FollowersChecker followersChecker = new FollowersChecker(Settings.EMPTY, transportService, fcr -> {
             assert false : fcr;
         }, (node, reason) -> {