StackRendererTest should restet the model after each test

StackRendererTest should remove the used window before each test
Before this change the the model
elements were not removed after a test.

tests/org.eclipse.e4.ui.tests/src/org/eclipse/e4/ui/workbench/renderers/swt/StackRendererTest.java

@@ -57,6 +57,7 @@
 import org.eclipse.swt.widgets.Event;
 import org.eclipse.swt.widgets.Label;
 import org.eclipse.swt.widgets.Widget;
+import org.junit.After;
 import org.junit.Before;
 import org.junit.Rule;
 import org.junit.Test;
@@ -84,11 +85,16 @@ public class StackRendererTest {
 	@Before
 	public void setUp() throws Exception {
 		window = ems.createModelElement(MWindow.class);
+		partStack = ems.createModelElement(MPartStack.class);
+		window.getChildren().add(partStack);
 		application.getChildren().add(window);
 		application.setSelectedElement(window);
+	}
 
-		partStack = ems.createModelElement(MPartStack.class);
-		window.getChildren().add(partStack);
+	@After
+	public void cleanUp() throws Exception {
+		ems.deleteModelElement(partStack);
+		ems.deleteModelElement(window);
 	}
 
 	@Test