Do not run mjsunit/compare-known-objects-slow in GC-stress mode.

NOTRY=true

Review URL: https://codereview.chromium.org/1393713004

Cr-Commit-Position: refs/heads/master@{#31155}

test/mjsunit/mjsunit.status

@@ -299,6 +299,7 @@
   'regress/regress-165637': [SKIP],
   'regress/regress-2249': [SKIP],
   'regress/regress-4121': [SKIP],
+  'compare-known-objects-slow': [SKIP],
   # Tests taking too long
   'debug-stepout-scope-part8': [SKIP],
   'mirror-object': [SKIP],