[foozzie] Add flag experiment on correctness fuzzer for atomic gc-stress

NOTRY=true
TBR=hpayer@chromium.org

Bug: v8:7012
Change-Id: If8517c27871af17a979d2766cc7f21b94035caa8
Reviewed-on: https://chromium-review.googlesource.com/870550Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50658}

tools/foozzie/v8_foozzie.py

@@ -82,6 +82,7 @@ ADDITIONAL_FLAGS = [
   (0.1, '--stress-marking=100'),
   (0.1, '--stress-scavenge=100'),
   (0.1, '--stress-compaction-random'),
+  (0.1, '--random-gc-interval=2000'),
 ]
 
 # Timeout in seconds for one d8 run.