selftests: drv-net: so_txtime: only send test traffic to sch_etf

The ETF qdiscs drops traffic without a socket or txtime. Even with
parameter skip_sock_check regular traffic is affected by ETF.

This test ran fine when run manually in a pure software environment.
But with drv-net across two hosts tests fail as early as when calling
cfg.remote.deploy due to effectively losing connectivity.

Isolate the intended test traffic:
- mark that with SO_MARK 100
- install a regular permissive root prio qdisc for background traffic
- install the ETF qdisc as leaf
- install a filter that only directs SO_MARK 100 traffic to this leaf

Technically other high prio traffic will map onto this leaf based on
ToS band mapping too. But that is immaterial in practice.

Fixes: 5c6baef3885c ("selftests: drv-net: convert so_txtime to drv-net")
Signed-off-by: Willem de Bruijn <willemb@google.com>
Link: https://patch.msgid.link/20260808160129.890119-1-willemdebruijn.kernel@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

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