This will switch on various testing features in sanitizer builds and
for correctness fuzzer builds.

Currently we group enabling the slow-path runtime flag and allocation
timeout for atomic gc stress, but more features could be added in the
future.

This will enable gc fuzzer, clusterfuzz and correctness fuzzer to use
both slow-path and atomic gc stress in release sanitizer builds.

Bug: v8:6972
Change-Id: I5cade68241a8a0711f8a388be7e1543aab0035fa
Reviewed-on: https://chromium-review.googlesource.com/869932
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50656}