Without realm-eval, the toplevel references in loaded tests are not
garbage-collected and changes in the tests can affect the global
object.

TBR=ulan@chromium.org
NOTRY=true

Bug: v8:7379, v8:7376
Change-Id: Id4d451e38fe49c6531a07d541ad3c67298b6acf2
Reviewed-on: https://chromium-review.googlesource.com/890519
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50915}