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

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

BugLink: https://bugs.launchpad.net/bugs/1867903
[ Upstream commit 0d29169a708bf730ede287248e429d579f432d1d ]

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Paolo Pisati <paolo.pisati@canonical.com>