ENH Auto workdir fixture

Ping @davidkleiven @jinchang This is more or less yoinked from ASE.

It changes the directory of the test to automatically run in the tmp_path. This way, you can create files without passing the path to the filename, and it will still always work in the temporary directory. It's mostly a safeguard, so tests cannot accidentally write files all over the place. Makes it a little less bad if a test forgets to set the directory.