Database testing does not sample migrations for databases other than main

Currently, the database testing pipeline samples migrations and batched background migrations only for the main database.

This issue was noted by @pedropombeiro on gitlab-org/gitlab!98689 (comment 1111058624)

Deliverables:

• The testing pipeline is running for both main and ci
• Batched background migration testing uses the correct database connection (waiting on gitlab-org/gitlab!100050 (merged) to merge, already approved)
• The rspec tests for the gitlab-com-database-testing pipeline need to be fixed (in-progress fix in !114 (merged))