There's only few testing code required for embedders. Add testing
directly to the regular cppgc target to allow working around an ODR
violation issue with //v8:v8 and //v8:v8_for_testing, see bug.

This unblock Oilpan library builds in Chromium.

Bug: v8:11952, chromium:1056170
Change-Id: I0f0448faa0904b9ece58bd5a24c79c86ee55e2bf
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3008216
Auto-Submit: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Dan Elphick <delphick@chromium.org>
Reviewed-by: Dan Elphick <delphick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#75581}