test(insertion-suppression): real-pair check falls back to pinned fixture (slice of fork 2dfe0f0)

Path-scoped slice of fork commit 2dfe0f0: only the
insertion-suppression real-pair test and the pinned fixture it falls
back to. The same commit's harvest-pin.test.mjs and
mitigation-output-form.test.mjs changes belong to the verification-tools
slice (cnighswonger#276) and are not part of this PR.

Co-Authored-By: Claude opus-5 <noreply@anthropic.com>

Author: Gunther-Schulz