Run test pipeline on GitHub CI

[reedy]

There are some config files setup around Blubber et al, but none of it is being run (at all?) on GitHub, so it's impossible to see if your commits are passing your own test pipeline

Filed as part of https://phabricator.wikimedia.org/T292110

[miselico]

Since 06c4a15 tests are run as part of the github workflow. Besides, several tests which we had removed with the idea of making the code base more lean were put back into the repository.