selftests: fib_tests: Add test cases for IPv4/IPv6 FIB

Add test cases to check that IPv4 and IPv6 react to a netdev being
unregistered as expected.

Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Acked-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>