kselftest: bonding: add num_grat_arp test

TEST: num_grat_arp (active-backup miimon num_grat_arp 10)           [ OK ]
TEST: num_grat_arp (active-backup miimon num_grat_arp 20)           [ OK ]
TEST: num_grat_arp (active-backup miimon num_grat_arp 30)           [ OK ]
TEST: num_grat_arp (active-backup miimon num_grat_arp 50)           [ OK ]

Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>