tests: use an absolute path to get around '..' being invalid on a dead CWD Only FreeBSD seems to be this picky. Note that this explicit absolute-path `cd` exposes a defect in the test, in that we end up still inside the cwd-vanish repository, but that's not a regression in this change. Since we're in a code freeze, I'm doing the smallest thing possible to try and fix bugs on FreeBSD, rather than cleaning up the entire problem. I'll follow up with a more complete fix after the freeze.