selftests: forwarding: router_mpath_hash: Add a new selftest

Add a selftest that exercises the sysctl added in the previous patches.

Test that set/get works as expected; that across seeds we eventually hit
all NHs (test_mpath_seed_*); and that a given seed keeps hitting the same
NHs even across seed changes (test_mpath_seed_stability_*).

Signed-off-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Nikolay Aleksandrov <razor@blackwall.org>
Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://lore.kernel.org/r/20240607151357.421181-6-petrm@nvidia.com


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