selftests: bonding: add bonding prio option test

Add a test for bonding prio option. Here is the test result:

]# ./option_prio.sh
TEST: prio_test (Test bonding option 'prio' with mode=1 monitor=arp_ip_target and primary_reselect=0)  [ OK ]
TEST: prio_test (Test bonding option 'prio' with mode=1 monitor=arp_ip_target and primary_reselect=1)  [ OK ]
TEST: prio_test (Test bonding option 'prio' with mode=1 monitor=arp_ip_target and primary_reselect=2)  [ OK ]
TEST: prio_test (Test bonding option 'prio' with mode=1 monitor=miimon and primary_reselect=0)  [ OK ]
TEST: prio_test (Test bonding option 'prio' with mode=1 monitor=miimon and primary_reselect=1)  [ OK ]
TEST: prio_test (Test bonding option 'prio' with mode=1 monitor=miimon and primary_reselect=2)  [ OK ]
TEST: prio_test (Test bonding option 'prio' with mode=5 monitor=miimon and primary_reselect=0)  [ OK ]
TEST: prio_test (Test bonding option 'prio' with mode=5 monitor=miimon and primary_reselect=1)  [ OK ]
TEST: prio_test (Test bonding option 'prio' with mode=5 monitor=miimon and primary_reselect=2)  [ OK ]
TEST: prio_test (Test bonding option 'prio' with mode=6 monitor=miimon and primary_reselect=0)  [ OK ]
TEST: prio_test (Test bonding option 'prio' with mode=6 monitor=miimon and primary_reselect=1)  [ OK ]
TEST: prio_test (Test bonding option 'prio' with mode=6 monitor=miimon and primary_reselect=2)  [ OK ]

Signed-off-by: Liang Li <liali@redhat.com>
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>