[test] Skip flaky test.

BUG=v8:4572
LOG=n
NOTRY=true
TBR=rossberg@chromium.org, adamk@chromium.org

Review URL: https://codereview.chromium.org/1475943002

Cr-Commit-Position: refs/heads/master@{#32251}

test/mjsunit/mjsunit.status

@@ -281,6 +281,9 @@
   'harmony/arrow-rest-params': [PASS, NO_VARIANTS],
   'harmony/rest-params': [PASS, ['no_snap == True', NO_VARIANTS]],
 
+  # BUG(v8:4572).
+  'es6/classes-subclass-builtins': [PASS, ['no_snap == True', NO_VARIANTS]],
+
   # Slow tests.
   'copy-on-write-assert': [PASS, SLOW],
   'debug-scopes': [PASS, SLOW],