Add GHA support to moodle-docker

[stronk7]

Basically does the same than old Travis runs, with some differences:

• I've grouped the jobs into 3 categories (phpunit, behat, app).
• Each one has own setup/teardown/test shell script, instead of the
  previous "mammoth" setup and test scripts, that I've left unmodified
• Aiming to get the travis CI runs disabled because they are super slow
  (we only have 2 concurrent jobs allowed) and, while running, all the
  other moodlehq repos have to wait.
• I'll create an issue to remove travis support once this
  is accepted and working ok.
• Only change that I had to perform to existing stuff is raising the
  hardcoded sleep of 5 seconds to 10 seconds because I was getting some
  random failures with MySQL, needing more time to start.
• We should move to proper health-checks for database (Implement proper health-check for databases #160) containers soon,
  much like app containers (Implement healthcheck for app development container #131).

[stronk7]

You can see it passing @ https://github.com/stronk7/moodle-docker/actions/runs/849100015

And the very-last commit (36fe4b0) that makes this PR is running @ https://github.com/stronk7/moodle-docker/actions/runs/849335063

Ciao :-)

PS: Obviously... this PR only can run Travis tests, because GHA is not upstream, heh. ;-)

[stronk7]

Have amended the commit (4ce009b) to save setting up one variable... latest changes are being run now by Travis and https://github.com/stronk7/moodle-docker/actions/runs/849537749.

Ciao :-)

[vmdef]

Thank you @stronk7. The structure is quite clear and the comment makes it even easier to understand the code. Only one minor typo that does not "matter".

[stronk7]

minor typo that does not "matter".

Whops, will try to fix it on next PR, thanks!