selftests: bonding: add fail_over_mac testing

Add a test to check each value of bond fail_over_mac option.

Also fix a minor garp_test print issue.

Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Link: https://patch.msgid.link/20250910024336.400253-2-liuhangbin@gmail.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>