Create a CI job that test new migrations with a lot of meaningful data

As described in https://gitlab.com/gitlab-org/gitlab-ce/issues/32341#note_29903427, we should create a new CI job that:

1. Checkout the target branch
2. Run migrations
3. Populate the DB with a lot of data: gitlab-ce#28149 / https://gitlab.com/gitlab-org/gitlab-ce/compare/master...28149-improve-seed
4. Merge the branch under test
5. Run the new migrations
6. Report timings of the new migrations