unity-greeter fails to build from source on all arches

[Adam Conrad]

First noticed with the ppc64el port, but then I retried a clean build on amd64, and it fails the same way:

UBUNTU_MENUPROXY=0 top_srcdir=.. . xvfb-run -a ./unity-greeter-test

(./unity-greeter-test:27427): Gtk-WARNING **: cannot open display: :99
/usr/bin/xvfb-run: line 184: 27427 Trace/breakpoint trap (core dumped) DISPLAY=:$SERVERNUM XAUTHORITY=$AUTHFILE "$@" 2>&1
make[2]: *** [check] Error 133
make[2]: Leaving directory `/«PKGBUILDDIR»/tests'
make[1]: *** [check-recursive] Error 1
make[1]: Leaving directory `/«PKGBUILDDIR»'
dh_auto_test: make -j1 check returned exit code 2

Would be lovely to have someone look into what's regressed here.