[cppgc][unittests] Only expect guard pages support on 4k platforms.

Arm64 supports 16k and 64k OS pages, in which case the CPPGC doesn't use
guard pages.

Bug: v8:10808
Change-Id: I36efba687c50b348eda62e9f9094b57bd58b55b5
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2485494


Commit-Queue: Pierre Langlois <pierre.langlois@arm.com>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70664}