To git-annex-test and git-annex-testremote, add option to run tests under concurrency (-J). Many possible bugs are unique to the concurrent case, and it's the case I often use. While any bugs detected may be hard to reproduce, it's important to know whether there are concurrency-related bugs. Much of the trust in git-annex comes from its extensive test suite, but it's somewhat concerning to trust it with important data when the concurrency case is not tested at all.