Add extended state variable change into test plan

- Add missing test method for testing variable change events.
- This may also contribute to make tests more reliable when
  testing variable values.
- Should contribute to #76 also.

spring-statemachine-zookeeper/src/test/java/org/springframework/statemachine/zookeeper/ZookeeperStateMachineTests.java

@@ -374,6 +374,7 @@ public class ZookeeperStateMachineTests extends AbstractZookeeperTests {
 					.step()
 						.sendEvent(message, machine1)
 						.expectTransition(1)
+						.expectExtendedStateChanged(1)
 						.expectVariable("testVariable", "x1")
 						.and()
 					.build();