Write test output to temporary folder and make tests not interact so they can be run in parallel

[bhazelton]

Currently many files are written to folders in the repo. They are deleted if tests pass but if tests fail they can linger behind. We should really only be writing to temporary folders.

Also, tests fail if pytest is run in parallel (with the -n auto) flag, which makes running tests much slower. This is especially problematic since so many of the tests (especially in test_xrfi.py) are really slow. I think much of this is related to outputs being written to the same file names, so this is likely to be fixed by going to writing all outputs to temporary folders.