Run tests via GitHub Actions

[jonafato]

The automation/tests/ directory contains pytest test that we should run via GitHub Actions against pull requests to ensure that tests pass with changes.

Additionally (and maybe this should be its own issue), we should build the site with Render_Engine with each change to ensure that template or content changes at least continue to build properly and without error. (I don't have any thoughts on actual "tests" for the content and behavior of the actual build output at the moment, but I'm also not against it.)

Both of these can use paths to selectively run the proper test suites where appropriate.