add a github action for PRs that runs the tests

[Pomax]

Pretty trivial, I've set this up more times than I can count by now.

[RoelN]

Small note, on my machine I occasionally seem to get stuck after the first font has been tested:

 PASS  testing/node/source.code.pro.test.js
 FAIL  testing/node/athena.ruby.test.js (5.115 s)

This happens roughly 25% of the times I run npm run test. I haven't investigated it this is something local, but I'm putting it here as a heads up!

[Pomax]

jest should time out 10 seconds after starting a test: does it not?