EMBA infrastructure improvements for Emacs build testing.

EMBA infrastructure improvements for Emacs build testing.

* .gitlab-ci.yml: Use job templates and rules. Split tests into
fast/normal/slow. Make Docker images for each tested platform
(inotify, filenotify-gio, gnustep). Increase timeout.
* test/Makefile.in (check-lisp, check-net): Add new testing
targets.
* test/README: Document them.
* test/file-organization.org: Mention test/infra.
* test/infra/Dockerfile.emba: Add special Docker recipes for EMBA
testing.