selftests/net/fib_tests: update addr_metric_test for peer route testing

This patch update {ipv4, ipv6}_addr_metric_test with
1. Set metric of address with peer route and see if the route added
correctly.
2. Modify metric and peer address for peer route and see if the route
changed correctly.

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