selftests: net/forwarding: test purge of active DWRR classes

selftests: net/forwarding: test purge of active DWRR classes

Extend sch_ets.sh to add a reproducer for problematic list deletions when
active DWRR class are purged by ets_qdisc_change() [1] [2].

[1] https://lore.kernel.org/netdev/e08c7f4a6882f260011909a868311c6e9b54f3e4.1639153474.git.dcaratti@redhat.com/
[2] https://lore.kernel.org/netdev/f3b9bacc73145f265c19ab80785933da5b7cbdec.1754581577.git.dcaratti@redhat.com/

Suggested-by: Victor Nogueira <victor@mojatatu.com>
Signed-off-by: Davide Caratti <dcaratti@redhat.com>
Acked-by: Victor Nogueira <victor@mojatatu.com>
Link: https://patch.msgid.link/489497cb781af7389011ca1591fb702a7391f5e7.1755016081.git.dcaratti@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>