Add automated e2e testing

[pratikmallya]

Every merge to master should kick off a set of integration tests that would:

• open a PR on a fake repo with codeowners (maybe pratikmallya/test? Or just create a new dedicated repo for this)

• check that the action creates labels as expected

  • should label with existing labels for existing codeowners
  • should create new labels for new codeowners

• choose what framework/tools to use for this testing

  • would be nice if there's an existing framework that allows for this sort of testing