more testing with git-annex in the loop

[notestaff]

I use git-annex-testremote with examples/git-annex-remote-directory to do end-to-end testing with git-annex in the loop. Right now that remote implementation does not use elements of the protocol; e.g. it does not claim URLs. It would be good to extend it, or add another example, so that all protocol elements could be tested with git-annex in the loop. (And so there'd be usage examples for documentation purposes).