It looks like the data directory was created as expected:
$ find test/data/ test/data/ test/data/log test/data/torrc
Unfortunately ./run_tests.py --help isn't telling me about e.g. a verbosity level option to help give more of a clue as to what is going wrong :-(
Any more ideas?
What happens when you run '/path/to/tor -f /path/to/stem/test/data/torrc'? That's all that stem is doing to start tor. If it's failing like this then running tor itself should also be failing.