selftests: forwarding: Add a test for NH group stats

Add to lib.sh support for fetching NH stats, and a new library,
router_mpath_nh_lib.sh, with the common code for testing NH stats.
Use the latter from router_mpath_nh.sh and router_mpath_nh_res.sh.

The test works by sending traffic through a NH group, and checking that the
reported values correspond to what the link that ultimately receives the
traffic reports having seen.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Link: https://lore.kernel.org/r/2a424c54062a5f1efd13b9ec5b2b0e29c6af2574.1709901020.git.petrm@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>