selftest: net: add proc_net_pktgen

Add some test for /proc/net/pktgen/... interface.

- enable 'CONFIG_NET_PKTGEN=m' in tools/testing/selftests/net/config

Signed-off-by: Peter Seiderer <ps.report@gmx.net>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>