selftests: rtnetlink: Add a test case for multipath route get

Without previous patch a warning would be generated upon multipath route
get when FIB multipath hash policy is to use a 5-tuple for multipath
hash calculation.

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