Disable a flaky test

This test starts failing when the --harmony-array flag is turned on,
but the failure does not directly have to do with that flag. Disabling
the test in debug mode to unblock the release.

BUG=v8:4237
LOG=n
R=adamk,erikcorry

Review URL: https://codereview.chromium.org/1202523005

Cr-Commit-Position: refs/heads/master@{#29207}

test/mjsunit/mjsunit.status

@@ -295,6 +295,9 @@
   'regress/regress-3717': [SKIP],
   # Issue 478788.
   'es7/object-observe': [SKIP],
+
+  # BUG(v8:4237)
+  'regress/regress-3976': [SKIP],
 }],  # 'gc_stress == True'
 
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