gitlab: Extract default build/test jobs templates

To be able to reuse the mainstream build/test jobs templates,
extract them into a new file (buildtest-template.yml).

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Willian Rampazzo <willianr@redhat.com>
Message-Id: <20210519185504.2198573-8-f4bug@amsat.org>
[thuth: Keep the "acceptance_test_job_template" name for now]
Signed-off-by: Thomas Huth <thuth@redhat.com>