selftests: router_mpath_nh: Test 16-bit next hop weights

Add tests that exercise full 16 bits of NH weight.

To test the 255:65535, it is necessary to run more packets than for the
other tests. On a debug kernel, the test can take up to a minute, therefore
avoid the 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/c0c257c00ad30b07afc3fa5e2afd135925405544.1723036486.git.petrm@nvidia.com


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