Test jobs with single db, two connections

What does this MR do and why?

Adds new variant for single db with two connections (one for main, one for ci)

We start with with database, migration specs, and background migration specs as this is where it typically differs with decomposed/single-db

Part of #391646 (closed)

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

You can see the expected config/database.yml is generated (I added a test cat config/database.yml):

https://gitlab.com/gitlab-org/gitlab/-/jobs/3796168645

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.