selftests: fib_tests: Add IPv6 nexthop spec tests

Add series of tests for valid and invalid nexthop specs for IPv6.

$ TEST=fib_nexthop_test ./fib_tests.sh
...
IPv6 nexthop tests
    TEST: Directly connected nexthop, unicast address              [ OK ]
    TEST: Directly connected nexthop, unicast address with device  [ OK ]
    TEST: Gateway is linklocal address                             [ OK ]
    TEST: Gateway is linklocal address, no device                  [ OK ]
    TEST: Gateway can not be local unicast address                 [ OK ]
    TEST: Gateway can not be local unicast address, with device    [ OK ]
    TEST: Gateway can not be a local linklocal address             [ OK ]
    TEST: Gateway can be local address in a VRF                    [ OK ]
    TEST: Gateway can be local address in a VRF, with device       [ OK ]
    TEST: Gateway can be local linklocal address in a VRF          [ OK ]
    TEST: Redirect to VRF lookup                                   [ OK ]
    TEST: VRF route, gateway can be local address in default VRF   [ OK ]
    TEST: VRF route, gateway can not be a local address            [ OK ]
    TEST: VRF route, gateway can not be a local addr with device   [ OK ]

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