selftests: forwarding: Parametrize mausezahn delay

The various multipath tests use mausezahn to generate different flows
and check how they are distributed between the available nexthops. The
tool is currently invoked with an hard coded transmission delay of 1 ms.
This is unnecessary when the tests are run with veth pairs and
needlessly prolongs the tests.

Parametrize this delay and default it to 0 us. It can be overridden
using the forwarding.config file. On my system, this reduces the run
time of router_multipath.sh by 93%.

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Link: https://lore.kernel.org/r/20240304095612.462900-3-idosch@nvidia.com


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