CppEditor: Tests: Use QTemporaryDir and avoid writing into creator source tree

This should improve results of CI tests where multiple creator instances
execute the plugin tests.

Change-Id: I557a0964568655662108df201589ba369096f4bf
Reviewed-by: Orgad Shaneh <orgads@gmail.com>
Reviewed-by: Erik Verbruggen <erik.verbruggen@theqtcompany.com>
Reviewed-by: Christian Stenger <christian.stenger@theqtcompany.com>

src/plugins/cppeditor/cppquickfix_test.h

@@ -82,6 +82,8 @@ protected:
     QList<QuickFixTestDocument::Ptr> m_testDocuments;
 
 private:
+    QScopedPointer<CppTools::Tests::TemporaryDir> m_temporaryDirectory;
+
     CppTools::CppCodeStylePreferences *m_cppCodeStylePreferences;
     QByteArray m_cppCodeStylePreferencesOriginalDelegateId;