When do autolaunch testing, libdbus will try to start dbus-launch in
installed direcotry, if fail then fall back to dbus-launch in $PATH.
dbus-launch does a relative better thing to start dbus-daemon in build
directory, however, in most of case, the build $prefix is different from
the real prefix where dbus-daemon installed. So dbus-daemon will fail to
start due to can't find its config file. And then dbus-launch will fall
back to finally the installed dbus-daemon.

This patch fix this behavior and will start dbus-launch and dbus-daemon
in build directory in test environment.

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=37849