Make it easier for tests to init the platform

Only a single static method is needed now, instead of 2.

src/qtquick/Platform_qtquick.h

@@ -33,8 +33,8 @@ public:
     QSize screenSizeFor(View *) const override;
 
 #ifdef DOCKS_DEVELOPER_MODE
-    void tests_initTests() override;
-    void tests_cleanupTests() override;
+    void tests_initPlatform_impl() override;
+    void tests_deinitPlatform_impl() override;
 #endif
 };