This enables the unit tests to run on any given backend, via the
`-remote` flag, e.g. `go test -v ./cmd/gitannex/... -remote dropbox:`.
We should also port the gitannex e2e tests at some point.
Issue: #7984
This commit adds support for the same repo layouts supported by
git-annex-remote-rclone. This should enable git-annex users with remotes
of type "rclone" to switch to a "rclone-builtin" without needing to
retransfer content.
Issue #7625
