Do not output to files when running tests

[AdrianDAlessandro]

On another note, I've noticed that when you run the tests you end up with new output files being written to tests/data/Supplementary/PDF/ (i.e. the same place as the input files) but they should be being written to the output directory, right? Also, specifically for tests, you generally want to write your output files to a temporary directory so it's a clean slate. The easiest way to do this is with pytest's tmpdir fixture.

Originally posted by @alexdewar in #235 (comment)