[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.
Document: cubicweb-doc
Title: CubicWeb documentation
Author: Logilab
Abstract: Some base documentation for CubicWeb users and developpers
Section: Apps/Programming
Format: HTML
Index: /usr/share/doc/cubicweb-documentation/index.html
Files: /usr/share/doc/cubicweb-documentation/*.html