Streamline repo setup for solver and repo testing

[dcookspi]

There is a difference between the repo setup for the standard Repository tests (that use the make_repo() fixture function), and the Solver tests (that use the make_repo!() macro). That difference should probably be removed in future, it's a minor irritation when adding tests to both places.

Originally posted by @dcookspi in #1339 (comment)

[dcookspi]

Another comment that's related to this aspect of repo/solver's test setup:
#1339 (comment)