CI: Add test rtdv3 GHA workflow

This is not a voting workflow, but rather it replicates what
Jenkins does in GHA.
The idea is to have this workflow tested first before we deprecate
the global-jjb rtdv3 job and switch to GHA.

Issue-ID: CIMAN-33
Change-Id: I83742b2fb9d16785a6932c93a2c701f13527307b
Signed-off-by: Jessica Wagantall <jwagantall@linuxfoundation.org>