[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/lib/PY_VERSION/*-packages/cubicweb/server/
usr/lib/PY_VERSION/*-packages/cubicweb/hooks/
usr/lib/PY_VERSION/*-packages/cubicweb/sobjects/
usr/lib/PY_VERSION/*-packages/cubicweb/schemas/
usr/share/cubicweb/migration/