[ci] add simple job for Theta runner

Add job step in `.ecp-ci.yml` plus the right variables to launch CI job
on theta. Not that Theta has mom nodes, so it's a bit different.

Make use of only: variable to limit job definition depending on mirror
being used within ECP.
6 jobs for ecp_test in 8 minutes and 23 seconds (queued for 2 seconds)