* testing/lisp/test-org-attach-annex.el: `default-directory' should end
in a slash to function properly. This was causing test failures on
older versions of git-annex.
* testing/lisp/test-org-attach-annex.el: Move all org-attach tests that
use git-annex to this file, which can test for the presence of
git-annex. Prevents tests failing on systems where git-annex is not
installed.