firewall: run test from source directory

As with many previous commits, there's no point in installing this
simple script; running it from the git directory gives us faster
fix/test cycles.

Reviewed-by: Sjoerd Simons <sjoerd.simons@collabora.co.uk>
Signed-off-by: Simon McVittie <simon.mcvittie@collabora.co.uk>
Differential Revision: https://phabricator.apertis.org/D3766