selftests: forwarding: lib: Allow passing PID to stop_traffic()

Now that it is possible to schedule a deferral of stop_traffic() right
after the traffic is started, we do not have to rely on the %% magic to
kill the background process that was started last. Instead we can just give
the PID explicitly. This makes it possible to start other background
processes after the traffic is started without confusing the cleanup.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>