Prepare to run test suite on all (default) events

In order to do this, we'll need to switch the mercurius repo's CI config back to .gitlab-ci.yml (rather than builddebs.yml@repos/sre/wmf-debci).

In advance of that, this switches .gitlab-ci.yml over to builddebs.yml to maintain consistency with the current multi-suite build process, in addition to introducing a test-stage CI job that runs the server_test test suite.

The latter uses the same idiom as many other projects, where tests are run within a blubber-defined container image (so that, e.g., the same environment is reproducible locally).