Also fixes existing tests which were asserting the wrong behavior (that
setting writable=false won't have an effect).

The bug was introduced by https://chromium-review.googlesource.com/c/v8/v8/+/1442640 .

Bug: chromium:1158138
Change-Id: I2d85721848eb4e7d530a980a9ecef7f8693bb9a2
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2691050
Commit-Queue: Marja Hölttä <marja@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72948}