Shift checkpoint files from test_chkpt to tmp

[Chiffafox]

Right now some of the tests -- specifically, test_chkpt -- are saving some files to disk in the head directory, then deleting them with shutil.rmtree. The proper way of handling this is is with a tmp_path.

[matthewcarbone]

@Chiffafox yep there's even a standard library class for this. It's really great when used in a context manager.