db gitserver repos: remove redundant test data and clean up tests a bit

Created by: mrnugget

Some of the things I found yesterday while looking at the tests.

• Some assertions are made more stable (instead of using anonymous counts it makes sure that the repos that we expect are iterated).
• Test helpers are reduced so the test doesn't include a lot of redundant data that has nothing to do with the test.
• Instead of upserting gitserver_repos only the column we're interested in is updated (since the DB trigger ensures a gitserver_repo is inserted).

Test plan

• These tests