Refactor away TemporaryCloneOfTestRepo

Following up on this conversation: https://github.com/libgit2/libgit2sharp/pull/312#discussion-diff-3514075

Before merging I'd suggest this be squashed into a single commit. For consistency usage, I'd suggest we either go with the approach suggested by the third commit (a `Clone___TestRepo()` corresponding to each test repo), or replace `CloneBareTestRepo()` with `Clone(BareTestRepoPath)`. I think I prefer the former (thus including it here).