selftests: net: packetdrill: xfail all problems on slow machines

We keep seeing flakes on packetdrill on debug kernels, while
non-debug kernels are stable, not a single flake in 200 runs.
Time to give up, debug kernels appear to suffer from 10msec
latency spikes and any timing-sensitive test is bound to flake.

Reviewed-by: Willem de Bruijn <willemb@google.com>
Acked-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20250801181638.2483531-1-kuba@kernel.org


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