selftests: router_mpath_nh_res: Test 16-bit next hop weights

Add tests that exercise full 16 bits of NH weight.

Like in the previous patch, omit the 255:65535 test when KSFT_MACHINE_SLOW.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://patch.msgid.link/a91d6ead9d1b1b4b7e276ca58a71ef814f42b7dd.1723036486.git.petrm@nvidia.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>