Fix X11 tests broken by the removal of -ac

Probably missed a few. Also adding xauth to the system path (it was
already in the closure).

nixos/lib/test-driver/Machine.pm

@@ -543,7 +543,7 @@ sub waitForX {
         retry sub {
             my ($status, $out) = $self->execute("journalctl -b SYSLOG_IDENTIFIER=systemd | grep 'session opened'");
             return 0 if $status != 0;
-            ($status, $out) = $self->execute("xwininfo -root > /dev/null 2>&1");
+            ($status, $out) = $self->execute("[ -e /tmp/.X11-unix/X0 ]");
             return 1 if $status == 0;
         }
     });