Disable zygote-jit-deopt under gcstress while investigating timeouts.

Test: zygote-jit-deopt
Bug: 137887811
Change-Id: Idde7e64eef910c65719d1274c3193eb90cee91b3
diff --git a/test/knownfailures.json b/test/knownfailures.json
index 049ff0d..0134a54 100644
--- a/test/knownfailures.json
+++ b/test/knownfailures.json
@@ -1267,5 +1267,11 @@
         "env_vars": {"ART_READ_BARRIER_TYPE": "TABLELOOKUP"},
         "bug": "b/140507091",
         "description": ["Test containing Checker assertions expecting Baker read barriers."]
+    },
+    {
+        "tests": ["689-zygote-jit-deopt"],
+        "variant": "gcstress",
+        "bug": "b/137887811",
+        "description": ["Occasional timeouts."]
     }
 ]