selftests: router_mpath: Sleep after MZ

In the context of an offloaded datapath, it may take a while for the ip
link stats to be updated. This causes the test to fail when MZ_DELAY is too
low. Sleep after the packets are sent for the link stats to get up to date.

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/8b1971d948273afd7de2da3d6a2ba35200540e55.1723036486.git.petrm@nvidia.com


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