cppgc/testing.h is already part of a testonly gn target which only can
be included from other test targets. This prevents any production
target to depend on cppgc/testing.h.

Bug: chromium:1056170
Change-Id: I51f6c47ffac2a05c8c63d7b4663c456a64fe75b4
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2689196Reviewed-by: Omer Katz <omerkatz@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72670}