chore: setup GitHub Actions workflow for continuous integration

[slarse]

#3600

Adds GitHub workflows for tests. The following is adapted from the Travis jobs:

• mvn test with JDK 1.8, 11 and 15
• Coverage uploading to coveralls
• The extra checks in travis-extras.sh, except for the external CI stuff

The following is not adapted from the Travis jobs:

• Running of the external CI jobs in https://github.com/SpoonLabs/spoon-ci-external, as they depend on Travis
  • This is simply skipped in travis-extra.sh as the environment variables don't match for the final if-check
• The chore/travis/travis-master.sh script, as it depends on Travis
  • This script isn't executed at all, it appears to post data to spoon-ci-external.

In regards to #3691, it would probably be simple to add a Windows build to the workflow as well.

I have not removed the Travis stuff, I'm assuming that we'll use GH Actions as a backup for when Travis becomes unavailable.

[slarse]

Ping @monperrus

Took me a while to get coverage uploading to work, but now all of the most important tests and checks are running on GH Actions.

[slarse]

I realized something: the new gha-extras.sh is essentially a copy of travis-extras.sh, with the final part of it removed. I'm gonna try running with travis-extras.sh instead, to reduce duplication. It should work as-is.

[slarse]

Sorry, unping Martin, I had missed some things.

[slarse]

@monperrus Okay, now it's done!

[monperrus]

Thanks a lot LGTM.

[monperrus]

Thanks a lot @slarse now we have CI again!

[slarse]

Great!