selftests: fib_tests: Add prefix route tests with metric

Add tests verifying prefix routes are inserted with expected metric.

IPv6 prefix route tests
    TEST: Default metric                                      [ OK ]
    TEST: User specified metric on first device               [ OK ]
    TEST: User specified metric on second device              [ OK ]
    TEST: Delete of address on first device                   [ OK ]
    TEST: Modify metric of address                            [ OK ]
    TEST: Prefix route removed on link down                   [ OK ]
    TEST: Prefix route with metric on link up                 [ OK ]

IPv4 prefix route tests
    TEST: Default metric                                      [ OK ]
    TEST: User specified metric on first device               [ OK ]
    TEST: User specified metric on second device              [ OK ]
    TEST: Delete of address on first device                   [ OK ]
    TEST: Modify metric of address                            [ OK ]
    TEST: Prefix route removed on link down                   [ OK ]
    TEST: Prefix route with metric on link up                 [ OK ]

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