[devtools] make xvfb-run not wait 3s each time (closes #2265710) Ask Xvfb to tell us when it's ready instead of unconditionally waiting 3 seconds. Sync from http://anonscm.debian.org/gitweb/?p=pkg-xorg/xserver/xorg-server.git;a=blob_plain;f=debian/local/xvfb-run;hb=HEAD