Unify test repository fixtures and make customization easier

[ecraig12345]

Currently includes #753

Unify the singe-repo, monorepo, and multi-monorepo factories into one repository factory.

Add the ability to pass in a custom fixture, to simplify setup and massively reduce boilerplate in tests that don't use the default fixtures.

Create all the fixture files in a single commit, which hopefully will make tests run slightly faster.