add .gitlab-ci-coverage.yml for a scheduled build

add .gitlab-ci-coverage.yml for a scheduled build

This will be used by the https://gitlab.com/samba-team/samba
configuration, while https://gitlab.com/samba-team/devel/samba
will still use .gitlab-ci.yml (via the legacy .gitlab-ci-private.yml).

The key point is the usage of the more powerful n1-standard-2
runners for testing.

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Andreas Schneider <asn@samba.org>
Autobuild-User(master): Stefan Metzmacher <metze@samba.org>
Autobuild-Date(master): Tue Apr 13 09:33:14 UTC 2021 on sn-devel-184