[weakrefs] Add missing extension to test file

Change-Id: I1a7afc332dc2f1c4d4087650f116484437f7a09a
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2285851
Auto-Submit: Shu-yu Guo <syg@chromium.org>
Commit-Queue: Marja Hölttä <marja@chromium.org>
Reviewed-by: Marja Hölttä <marja@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68731}