[devtools] fix race when creating backupdir If somebody else creates backupdir between our stat() and mkdir() (like, say, another test running in parallel), we shouldn't crash.

usr/bin/cubicweb-ctl usr/bin/
usr/lib/PY_VERSION/*-packages/cubicweb/cwctl.py
../cubicweb-ctl.bash_completion etc/bash_completion.d/cubicweb-ctl