Fix QQuickTextInput test instability.

Use a QWindow to steal focus rather than a second QQuickView, as it
appears there may be some issue with shared GL resources between views.

Change-Id: I8a134e3b4a1e54f645d0071543ec8f94338ff315
Reviewed-by: Yann Bodson <yann.bodson@nokia.com>

tests/auto/quick/qquicktextinput/tst_qquicktextinput.cpp

@@ -2402,7 +2402,7 @@ void tst_qquicktextinput::cursorVisible()
     QCOMPARE(input.isCursorVisible(), true);
     QCOMPARE(spy.count(), 5);
 
-    QQuickView alternateView;
+    QWindow alternateView;
     alternateView.show();
     alternateView.requestActivateWindow();
     QTest::qWaitForWindowShown(&alternateView);