Make it easier to run python tests

[pmolodo]

At the moment, the only source of the "complete" set of python tests seems to be in .github/workflows/main.yml, under the Python Tests step.

It would be nice if this were extracted into it's own standalone script, or similar, to enable easier running outside of github. Also, I'm not clear on how github detects failure for many of these tests - ie, Scripts/generateshader.py seems to swallow many errors, and return 0, so I was forced to scrape output for Failed strings.

[jstone-lucasfilm]

These are good suggestions, @pmolodo, and I'll note that the error reporting in generateshader should be improved by the recent merge of #1160.