[test] DialogCheck: reuse VerifyDialog

to reduce used OS handles
try to avoid "org.eclipse.swt.SWTError: No more handles" during tests

Author: jukzi

tests/org.eclipse.ui.tests.harness/src/org/eclipse/ui/tests/harness/util/DialogCheck.java

@@ -104,8 +104,10 @@ public static void assertDialogTexts(Dialog dialog) {
 	public static Shell getShell() {
 		Shell shell = PlatformUI.getWorkbench()
 				.getActiveWorkbenchWindow().getShell();
-		_verifyDialog = new VerifyDialog(shell);
-		_verifyDialog.create();
+		if (_verifyDialog == null) {
+			_verifyDialog = new VerifyDialog(shell);
+			_verifyDialog.create();
+		}
 		return _verifyDialog.getShell();
 	}