selftests/tc-testing: add hhf hh_limit cap tests

Cover the new TCA_HHF_HH_FLOWS_LIMIT bound: values above 2*HH_FLOWS_CNT
(4294967295, 65536, 2049) are rejected with the configured limit left
untouched on both the change and the add path, the boundary value 2048 is
accepted (installed at 100 first so the boundary change is load-bearing),
and an add-time hh_limit 500 is preserved instead of being clobbered by
the default.

Reviewed-by: Victor Nogueira <victor@mojatatu.com>
Tested-by: hybris <hybris@mojatatu.ai>
Signed-off-by: Jamal Hadi Salim <jhs@mojatatu.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/QDISC-B855.v1.20260911153152@mojatatu.com.2
Signed-off-by: Paolo Abeni <pabeni@redhat.com>