Add NSEC3 excessive iterations test to dnssec_py

Add NSEC3 excessive iterations test to dnssec_py

Rewrite nsec3_delegation/tests_excessive_nsec3_iterations.py as
dnssec_py/tests_nsec3_iter_too_many.py using the isctest.zone helpers.

The test is a reproducer for CVE-2026-1519 [GL#5708]. It sets up a
delegation from nsec3-iter-too-many. (ns2) to an unsigned sub zone
(ns3), signing the parent with NSEC3 at 51 iterations. A validating
resolver (ns9) must use NSEC3 to prove the sub zone is insecure; the
excessive iteration count is logged as a warning. The test verifies that
the query still resolves successfully (insecure, not SERVFAIL) despite
the high iteration count.

Assisted-by: Claude:claude-opus-4-8
(cherry picked from commit fc5116ed9164c28a1c76c2a42f0f99834f47fc58)