And fix all the tests as we need to wait for the project to be loaded
to check the reference count of the timeline (as we keep a ref on the
timeline in project to later emit "loaded" on idle).
Those are test with real media files, they are run separetely from other
unit tests using the make check-integration command (can be done from
the toplevel directory)
Check if an object rthat has already been freed has been destroyed is not safe.
Add a helper function that uses weak reference to check that objects that are expected
to be destroyed when unrefing an object are actually destroyed.
