Add basic tests. Run the ssh-mode test on GitHub Actions.

[lgarrison]

This PR adds a basic test, which touches some files then checks that those files exist. There are two copies of this test: one for ssh mode, and one for Slurm. While for now the only thing different about the tests is the disBatch invocation, I've kept them separate because we might want to add more functionality to the Slurm test in the future. The ssh-mode is tested on GitHub Actions for Python versions 3.6 to 3.10.

The Slurm test can only be run locally for now, but in theory it would be possible to set up a GitHub Action that installs Slurm and runs the test.

[njcarriero]

Looks good. Two quick questions:

• Why not include python 3.11 in matrix.python-version?
• I assume the github action execution environment is ephemeral, so no need to clean up the temp dir?

[lgarrison]

Sure, I'll add Python 3.11. I haven't been including it in my other projects because many upstream dependencies don't support it yet, but disBatch doesn't have any upstream dependencies!

There's no need to clean anything up on the GitHub runners, although it currently doesn't clean up local temp dirs to aid debugging/manual inspection. Maybe I'll have it clean up after itself on a successful run though...