[test] Fix MockMutableAuthenticationState implementation

When #19519 was
cherry-picked to branch-2.11, it did not implement the
authenticate method in the MockMutableAuthenticationState,
which led to several test failures in the ServerCnxTest class.
This commit fixes those tests.

Note that the issue is only in the test code.

Author: michaeljmarshall

pulsar-broker/src/test/java/org/apache/pulsar/broker/auth/MockMutableAuthenticationState.java

@@ -19,6 +19,7 @@
 package org.apache.pulsar.broker.auth;
 
 import static java.nio.charset.StandardCharsets.UTF_8;
+import java.util.concurrent.ExecutionException;
 import javax.naming.AuthenticationException;
 import java.util.concurrent.CompletableFuture;
 import org.apache.pulsar.broker.authentication.AuthenticationDataCommand;
@@ -47,7 +48,11 @@ public String getAuthRole() throws AuthenticationException {
 
     @Override
     public AuthData authenticate(AuthData authData) throws AuthenticationException {
-        return null;
+        try {
+            return this.authenticateAsync(authData).get();
+        } catch (InterruptedException | ExecutionException e) {
+            throw new RuntimeException(e);
+        }
     }
 
     /**