Delete the Platform when a test is complete

So that we support testing multiple platforms in the same test run

src/qtwidgets/Platform_qtwidgets.cpp

@@ -108,11 +108,16 @@ QSize Platform_qtwidgets::screenSizeFor(View *view) const
 
 #ifdef DOCKS_DEVELOPER_MODE
 
-void Platform_qtwidgets::tests_initTests() const
+void Platform_qtwidgets::tests_initTests()
 {
     qputenv("KDDOCKWIDGETS_SHOW_DEBUG_WINDOW", "");
     Platform_qt::tests_initTests();
     qApp->setStyle(QStyleFactory::create(QStringLiteral("fusion")));
 }
 
+void Platform_qtwidgets::tests_cleanupTests()
+{
+    Platform_qt::tests_cleanupTests();
+}
+
 #endif