1.x: Deprecate TestObserver, enhance TestSubscriber a bit (#4011)

Author: akarnokd

src/main/java/rx/observers/TestObserver.java

@@ -24,7 +24,9 @@
 /**
  * Observer usable for unit testing to perform assertions, inspect received events or wrap a mocked Observer.
  * @param <T> the observed value type
+ * @deprecated use the {@link TestSubscriber} insteand.
  */
+@Deprecated
 public class TestObserver<T> implements Observer<T> {
 
     private final Observer<T> delegate;