tests: Add test for 'ip route add default'

Signed-off-by: Vadim Kochan <vadim4j@gmail.com>

testsuite/tests/ip/route/add_default_route.t

+#!/bin/sh
+
+source lib/generic.sh
+
+ts_log "[Testing add default route]"
+
+DEV=dummy0
+
+ts_ip "$0" "Add new interface $DEV" link add $DEV type dummy
+ts_ip "$0" "Set $DEV into UP state" link set up dev $DEV
+ts_ip "$0" "Add 1.1.1.1/24 addr on $DEV" addr add 1.1.1.1/24 dev $DEV
+ts_ip "$0" "Add default route via 1.1.1.1" route add default via 1.1.1.1