qa/run_xfstests.sh: use old xfstests until we adapt to new org

Tests were rearranged upstream; use an old version for the time being
until we can refactor run_xfstests.sh to cope.  See #6385

Signed-off-by: Sage Weil <sage@inktank.com>
Merge pull request #646 from liewegas/wip-6385

qa/run_xfstests.sh: use old xfstests until we adapt to new org

Reviewed-by: Josh Durgin <josh.durgin@inktank.com>