Add CI support to the exttests repository

Was looking to some PRs in the external tests repository:

https://github.com/moodlehq/moodle-exttests

When I thought that it wouldn't be crazy to have the repo CI-coverered. That way stuff could be accepted in a safer way and enable us to detect potential problems in advance. After all, the repository (deployed @ downloads) is used by everybody in the world running our unit tests, so some stability is welcome.

Basically I see it like something achievable in travis:

1. Publish the repo.
2. Prepare a Moodle phpunit run
3. Configure it to use the published ext tests in #1.
4. Verify unit tests pass (multiple branches...)

Dropping the idea here to avoid forgetting it... ciao