MAINT: `run_tests.py` should have `--debug` option

[larsoner]

It would be nice to do python tests/run_tests.py ds000248 --debug and have it

1. Automatically set n_jobs=1
2. Set on_error='debug'

I went to do this but it quickly became complicated because failsafe_run requires on_error as an input, and every script imports on_error from config... so we'd have to add a get_on_error like we have for get_n_jobs or something. Not entirely sure...

One question is, why is on_error a parameter for failsafe_run at all? It seems like it should just use the on_error already in the config.py namespace where it's defined.