Add automated integration tests

[20k-ultra]

We need integration tests which run the whole action to avoid regressions like #111 that use the action.yml which we can't test since that file is used for the action setup.

We have the npm script to run an action given a .env file and event.json however this doesn't really run an action it just pretends to be the action runner so if the action runner logic changes these scripts won't match it.