These tests are ok but could be modernised a bit

- use `safeAwait` instead of awaiting the latch directly
- move assertions into handler, avoiding the need for `responseHolder`
- would it be better to use an `ActionListenerResponseHandler` rather than a raw `TransportResponseHandler`?
server/src/test/java/org/elasticsearch/action/admin/cluster/configuration/TransportAddVotingConfigExclusionsActionTests.java
import org.elasticsearch.cluster.coordination.CoordinationMetadata.VotingConfiguration;
import org.elasticsearch.cluster.coordination.Reconfigurator;
import org.elasticsearch.cluster.metadata.Metadata;
import org.elasticsearch.cluster.node.DiscoveryNode;
import org.elasticsearch.cluster.node.DiscoveryNodeRole;
import org.elasticsearch.cluster.node.DiscoveryNodeUtils;
import org.elasticsearch.cluster.node.DiscoveryNodes.Builder;
import org.elasticsearch.cluster.service.ClusterService;
import org.elasticsearch.common.Strings;
import org.elasticsearch.common.io.stream.StreamInput;
import org.elasticsearch.common.settings.ClusterSettings;
import org.elasticsearch.common.settings.Settings;
import org.elasticsearch.core.TimeValue;
import org.elasticsearch.indices.TestIndexNameExpressionResolver;
import org.elasticsearch.test.ESTestCase;
import org.elasticsearch.test.transport.MockTransport;
import org.elasticsearch.threadpool.TestThreadPool;
import org.elasticsearch.threadpool.ThreadPool;
import org.elasticsearch.transport.TransportException;
import org.elasticsearch.transport.TransportResponseHandler;
import org.elasticsearch.transport.TransportService;
import org.junit.AfterClass;
import org.junit.Before;
import org.junit.BeforeClass;
import java.util.HashSet;
import java.util.Set;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.Executor;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.atomic.AtomicBoolean;