[devtools] fix a couple issues with xvfb-run
xvfb-run didn't quite clean up correctly after itself, because:
- dash doesn't run EXIT traps on signals
- if we don't run the command in the background, a TERM handler doesn't run
until the command exits
debian/tmp/usr/lib/PY_VERSION/site-packages/cubicweb/web usr/lib/PY_VERSION/site-packages/cubicweb
debian/tmp/usr/share/cubicweb/cubes/shared/data usr/share/cubicweb/cubes/shared
debian/tmp/usr/share/cubicweb/cubes/shared/wdoc usr/share/cubicweb/cubes/shared