[testing] Add gc_stats variant

This is to allow testing object stats collection.

No-try: true
Bug: v8:7760
Change-Id: I1e5318f18bfb3799b90c0ee0aec6df87a794b32c
Reviewed-on: https://chromium-review.googlesource.com/1065478
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53274}

tools/testrunner/local/variants.py

@@ -7,6 +7,7 @@ ALL_VARIANT_FLAGS = {
   "code_serializer": [["--cache=code"]],
   "default": [[]],
   "future": [["--future"]],
+  "gc_stats": [["--gc_stats=1"]],
   # Alias of exhaustive variants, but triggering new test framework features.
   "infra_staging": [[]],
   "liftoff": [["--liftoff"]],