lookup: remove underscore flaky for s390 (#949)

Author: pacostas

lib/lookup.json

@@ -521,7 +521,7 @@
     "comment": "Tests timeout"
   },
   "underscore": {
-    "flaky": ["aix", "s390"],
+    "flaky": ["aix"],
     "skip": "win32",
     "maintainers": "jashkenas"
   },