C++: Fix resolving ui_* files in CppPreprocessor

The working copy contains the artificial ui_* files and therefore we
have to consider it while resolving include files.

Task-number: QTCREATORBUG-9683
Change-Id: Icb3387b4cd885b3652bae3f1da465d3e0f633332
Reviewed-by: Christian Stenger <christian.stenger@digia.com>
Reviewed-by: Eike Ziller <eike.ziller@digia.com>
Reviewed-by: Daniel Teske <daniel.teske@digia.com>
Reviewed-by: Robert Loehning <robert.loehning@digia.com>
Reviewed-by: Erik Verbruggen <erik.verbruggen@digia.com>

src/plugins/cpptools/modelmanagertesthelper.h

@@ -79,7 +79,7 @@ public:
     ~ModelManagerTestHelper();
 
     void cleanup();
-    void verifyClean();
+    static void verifyClean();
 
     Project *createProject(const QString &name);