pktgen: unit_test01 add proc writes

I nterface config changes (via proc) can occur while pktgen is
running, which worries me a bit.  I'm assuming proc_remove() takes
appropriate locks, to assure no writers exists after proc_remove()
finish.

Add testing this race to the script too.

Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>