tests: Add tests for DockWidget and FloatingWindow ctor

To catch potential QML errors in the .qml files.

src/qtquick/views/DockWidget_qtquick.cpp

@@ -59,8 +59,6 @@ DockWidget_qtquick::DockWidget_qtquick(Controllers::DockWidget *controller,
     : View_qtquick(controller, Type::DockWidget, nullptr, windowFlags)
     , d(new Private(this, controller, engine ? engine : plat()->qmlEngine()))
 {
-    // To mimic what QtWidgets does when creating a new QWidget.
-    setVisible(false);
 }
 
 DockWidget_qtquick::~DockWidget_qtquick()
@@ -68,6 +66,12 @@ DockWidget_qtquick::~DockWidget_qtquick()
     delete d;
 }
 
+void DockWidget_qtquick::init()
+{
+    // To mimic what QtWidgets does when creating a new QWidget.
+    setVisible(false);
+}
+
 void DockWidget_qtquick::setWidget(const QString &qmlFilename)
 {
     QQuickItem *guest = createItem(d->m_qmlEngine, qmlFilename);