Pubsub test client fixup (#25907)

* make incoming message public

* add set clock method to test client

* spotless

sdks/java/io/google-cloud-platform/src/main/java/org/apache/beam/sdk/io/gcp/pubsub/PubsubTestClient.java

@@ -100,7 +100,11 @@ private static class State {
   private static final State STATE = new State();
 
   /** Closing the factory will validate all expected messages were processed. */
-  public interface PubsubTestClientFactory extends PubsubClientFactory, Closeable, Serializable {}
+  public interface PubsubTestClientFactory extends PubsubClientFactory, Closeable, Serializable {
+    default <T> PubsubIO.Read<T> setClock(PubsubIO.Read<T> readTransform, Clock clock) {
+      return readTransform.withClock(clock);
+    }
+  }
 
   /**
    * Return a factory for testing publishers. Only one factory may be in-flight at a time. The