[testlib] gather all repository access logic in one place
Refactoring of the repository access API in test is imminent. We plan to move
from the "old" dbapi to the new repoapi.
Gathering all impacted method in one place help to understand how all those
method interact and help readability for both patch and resulting code.
No code change is done at all in this changeset. The refactoring will code
later.