Create some workflow allow to test basic functionality

[stronk7]

While we use a lot of the functionality in our CI servers and we usually detect any regression quickly, we don't use all the functionalities all the time. See for example #79 or #80, where it has been reported that there are problems with the main/replica support built on moodle-ci-runner and nobody had noticed that.

So it will be interesting to be able to add some own CI (GHA) to this repository and have as much functionality as possible covered. Surely not needing to perform any complete phpunit/behat run at all, but at very least ensure that:

• all the containers are started properly,
• logs working,
• main/replica configurations doing their job
• phpunit and behat executing ok (using a small suite...)
• ... and other bits. The more the better.

Ciao :-)