Add description of the options for pytest in the docs

[jwreep]

I'd like to suggest a small improvement to the docs. The description of how to use the pytests is currently a bit limited. It would be helpful to have a page or short list explaining each option for pytest that can be used with fiasco. I was having a bit of trouble understanding some options for running the pytests.

For example,

pytest --ascii-dbase-url http://download.chiantidatabase.org/CHIANTI_v9.0.1_database.tar.gz --skip-version-check --disable-file-hash has three separate switches, but they're not explained in the docs. The first is self-explanatory, but it's not clear what the latter two do without digging into the code.

[wtbarnes]

I completely agree. This needs to be documented much better. I suspect you're the first person besides me to use any of these!

We should do two things:

1. Add some narrative docs to this section of the how-to guides: https://fiasco.readthedocs.io/en/stable/how_to_guides.html#run-the-test-suite
2. Document each custom argument in the conftest.py: https://github.com/wtbarnes/fiasco/blob/6642b07c4d56f8327ea30e4aaf20da96a4258498/conftest.py. I did it for the last argument (using the help=... kwarg). If we do this, these explanations show up in the "Custom options" section when you run pytest -h.