CppTools: Tests: Fix crash in test_modelmanager_gc_if_last_cppeditor_closed

The project that was referenced at cppeditor.cpp:711 was already
deleted. CppModelManager::m_fileToProjectParts contained outdated
information. Now it is updated if a project is closed.

Exposed by commit d411c076.

Change-Id: I6fee93a02c72a045dd44a05723ab39a1eeec616e
Reviewed-by: Erik Verbruggen <erik.verbruggen@digia.com>