Autotest parsing takes 100% CPU for too long

Project::files() is called for each open source file in TestCodeParser::onDocumentUpdated(). files() itself is O(n), so we have O(files_in_project*open_files), which is unacceptable for large projects (like qtc itself!).

To make things worse, TestCodeParser::emitUpdateTestTree() is called by SessionManager::startupProjectChanged, by CppModelManager::projectPartsUpdated and by ProgressManager::allTasksFinished. Each of them scans the entire project tree (unless they're called less than a second from the previous call)...

To observe this, place a breakpoint in Project::files().