selftests: net: Add bridge backup port and backup nexthop ID test

Add test cases for bridge backup port and backup nexthop ID, testing
both good and bad flows.

Example truncated output:

 # ./test_bridge_backup_port.sh
 [...]
 Tests passed:  83
 Tests failed:   0

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Acked-by: Nikolay Aleksandrov <razor@blackwall.org>
Signed-off-by: David S. Miller <davem@davemloft.net>