selftests: mlxsw: sharedbuffer: Ensure no extra packets are counted

selftests: mlxsw: sharedbuffer: Ensure no extra packets are counted

The test assumes that the packet it is sending is the only packet being
passed to the device.

However, it is not the case and so other packets are filling the buffers
as well. Therefore, the test sometimes fails because it is reading a
maximum occupancy that is larger than expected.

Add egress filters on $h1 and $h2 that will guarantee the above.

Fixes: a865ad999603 ("selftests: mlxsw: Add shared buffer traffic test")
Signed-off-by: Danielle Ratson <danieller@nvidia.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: Petr Machata <petrm@nvidia.com>
Link: https://patch.msgid.link/64c28bc9b1cc1d78c4a73feda7cedbe9526ccf8b.1733414773.git.petrm@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>