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>

selftests: forwarding: Remove IPv6 L3 multipath hash tests

The multipath tests currently test both the L3 and L4 multipath hash
policies for IPv6, but only the L4 policy for IPv4. The reason is mostly
historic: When the initial multipath test was added
(router_multipath.sh) the IPv6 L4 policy did not exist and was later
added to the test. The other multipath tests copied this pattern
although there is little value in testing both policies.

Align the IPv4 and IPv6 tests and only test the L4 policy. On my system,
this reduces the run time of router_multipath.sh by 89% because of the
repeated ping6 invocations to randomize the flow label.

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Link: https://lore.kernel.org/r/20240304095612.462900-2-idosch@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

selftests: forwarding: Add multipath tunneling nexthop test

Add a nexthop objects version of gre_multipath.sh. Unlike the original
test, it also tests IPv6 overlay which is not possible with the legacy
nexthop implementation. See commit 9a2ad3623868 ("selftests: forwarding:
gre_multipath: Drop IPv6 tests") for more info.

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>