Add a test for thread safety with concurrent get_available() and done() calls in TrackingTopologicalSorter

We recently added #795 , after merging it I realized that we should add a test around thread safety.

Test thread safety with concurrent get_available() and done() calls in https://github.com/python-wheel-build/fromager/blob/main/tests/test_dependency_graph.py. `TrackingTopologicalSorter(graph)` should work correctly with multiple threads.