Ignore test failure for mjsunit/for-in-opt in gc stress.

TBR=hablich@chromium.org
BUG=v8:4381

Review URL: https://codereview.chromium.org/1295513004

Cr-Commit-Position: refs/heads/master@{#30245}

test/mjsunit/mjsunit.status

@@ -322,6 +322,9 @@
 
   # BUG(v8:4359)
   'strong/load-proxy': [SKIP],
+
+  # BUG(v8:4381)
+  'for-in-opt': [PASS, FAIL],
 }],  # 'gc_stress == True'
 
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