Autotest: Move unit plugin test creation closer to the tested code

The current pattern.

Change-Id: I52354584734755b3b0f3d2c9596801dc050300fc
Reviewed-by: Jarek Kobus <jaroslaw.kobus@qt.io>

src/plugins/autotest/autotestplugin.cpp

@@ -511,7 +511,7 @@ public:
         ExtensionSystem::PluginManager::registerScenario("TestModelManagerInterface",
                        [] { return dd->m_loadProjectScenario(); });
 
-        addTest<AutoTestUnitTests>(&dd->m_testTreeModel);
+        addTestCreator(createAutotestUnitTests);
     #endif
     }