gitlab: Extract all default build/test jobs to buildtest.yml

Extract the build/test jobs run by default on the mainstream
CI into a new file (buildtest.yml).

Reviewed-by: Willian Rampazzo <willianr@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20210511072952.2813358-9-f4bug@amsat.org>