selftests/tc-testing: Add tests for restrictions on netem duplication

selftests/tc-testing: Add tests for restrictions on netem duplication

Ensure that a duplicating netem cannot exist in a tree with other netems
in both qdisc addition and change. This is meant to prevent the soft
lockup and OOM loop scenario discussed in [1]. Also adjust a HFSC's
re-entrancy test case with netem for this new restriction - KASAN
still triggers upon its failure.

[1] https://lore.kernel.org/netdev/8DuRWwfqjoRDLDmBMlIfbrsZg9Gx50DHJc1ilxsEBNe2D6NMoigR_eIRIG0LOjMc3r10nUUZtArXx4oZBIdUfZQrwjcQhdinnMis_0G7VEk=@willsroot.io/

Signed-off-by: William Liu <will@willsroot.io>
Reviewed-by: Savino Dicanosa <savy@syst3mfailure.io>
Acked-by: Jamal Hadi Salim <jhs@mojatatu.com>
Link: https://patch.msgid.link/20250708164219.875521-1-will@willsroot.io
Signed-off-by: Jakub Kicinski <kuba@kernel.org>