Dispose leftover shells in WorkbenchContextExtension cleanup

The dispose() method calls wb.close() which removes GUI for
application windows, but does not dispose shells created as side
effects (e.g. PartRenderingEngine's limbo shell). These accumulate
across test methods, causing many unexpected windows to open.

Explicitly dispose all remaining shells after wb.close() to ensure
a clean state between test methods.

See #3893

Author: vogella

tests/org.eclipse.e4.ui.tests/src/org/eclipse/e4/ui/tests/rules/WorkbenchContextExtension.java

@@ -30,6 +30,7 @@
 import org.eclipse.e4.ui.workbench.modeling.EModelService;
 import org.eclipse.e4.ui.workbench.swt.DisplayUISynchronize;
 import org.eclipse.swt.widgets.Display;
+import org.eclipse.swt.widgets.Shell;
 import org.junit.jupiter.api.extension.AfterEachCallback;
 import org.junit.jupiter.api.extension.BeforeEachCallback;
 import org.junit.jupiter.api.extension.ExtensionContext;
@@ -90,6 +91,15 @@ protected void dispose() {
 			wb.close();
 		}
 
+		// Dispose any remaining shells that were not cleaned up by wb.close(),
+		// such as PartRenderingEngine's limbo shell
+		Display display = Display.getDefault();
+		for (Shell shell : display.getShells()) {
+			if (!shell.isDisposed()) {
+				shell.dispose();
+			}
+		}
+
 		ContextInjectionFactory.setDefault(null);
 
 		// Workaround, enforce uninjection on the UIEventTopic provider from context