ok it looks like it's working! some notes:

• It needed to have some kind of real or fake interactive session -- it would work over ssh into the server, but not in github actions until I used xvfb-run
• If the CI environment variable is present, webdriverio adds some chrome flags. Even if we're setting --headless=new in flags manually, we also have to add headless: false to the webdriverio config to make it not add different headless flags :')
• We needed a few other flags, like --use-cmd-decoder=passthrough to get Chrome to use the real gpu and not try to use its seemingly currently nonfunctional software version

TODOs:

• Remove excess logging that I needed for debugging the setup
• Bring this into base processing/p5.js proper

Now I'm running this https://github.com/davepagurek/p5-ci-runner to spin up a new container for each test run to keep things isolated and secure