Add tests for tuf-repository

[mnm678]

Add tests for init and set up the testing infrastructure for future features.

[sudo-bmitch]

Two thoughts on this one. If you want to make it an ephemeral test, I would use a directory that isn't checked into git. There is a t.TempDir() you can use to make a new directory that Go always cleans for you. Docs on that one: https://pkg.go.dev/testing#T.TempDir

And for test data you do want to include in the project, there's an under documented folder name called testdata that Go always ignores. So I tend to keep any non-Go data in those directories. There are a couple hints in the Go docs on this but it should really be called out better than finding it in Dave's blog posts: https://dave.cheney.net/2016/05/10/test-fixtures-in-go

Neither of these are big enough to hold up the merge of these test, they LGTM.