Follow up to 3f4f4a0d: "[regexp] Handle zero-length matches in
experimental engine".

We want to assert an equivalence, not an implication in the boring
direction.

Bug: v8:10765
Change-Id: Ia7b4851feb1f5f621d687779d0cae0e28433f00e
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2398536Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Martin Bidlingmaier <mbid@google.com>
Cr-Commit-Position: refs/heads/master@{#69764}