Use pytest's tmp_path fixture instead of tempfile.NamedTemporaryFile
This PR improves the test suite (IMO) by replacing some usage of tempfile.NamedTemporaryFile with the tmp_path fixture from pytest. This is intended to make the tests more readable.
In the process I ended up refactoring the monolithic I/O tests for the hdf5.pycbc_live and hdf5.snax I/O formats.
cc @myNameIsPatrick for a review of the hdf5.snax test changes.