CompilationDatabase: Fix few bugs and plugin tests

- do not convert the DOS path if it does not exist
 - pass the proper flags to addDriverModeFlagIfNeeded
 - reset the kit toolchain before picking the matching one
 - do not show the popup when running plugin tests
 - clang++ contains 'g++' but it's not a gcc compiler

Change-Id: Ia5018a2131bb123202b87e34e0ac3a79f0feb45d
Reviewed-by: Christian Kandeler <christian.kandeler@qt.io>

src/plugins/compilationdatabaseprojectmanager/compilationdatabaseproject.h

@@ -58,6 +58,7 @@ private:
 
     void reparseProject();
     void buildTreeAndProjectParts(const Utils::FileName &projectFile);
+    Utils::FileName rootPathFromSettings() const;
 
     QFutureWatcher<void> m_parserWatcher;
     std::unique_ptr<CppTools::CppProjectUpdater> m_cppCodeModelUpdater;