If a Proxy is passed as argument to d8.test.verifySourcePositions,
unwrap the target function.

Bug: chromium:1186491
Change-Id: Iaacbf800f023121a07035a88745919df68f79217
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2748081Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Commit-Queue: Patrick Thier <pthier@chromium.org>
Cr-Commit-Position: refs/heads/master@{#73398}