Update objective-c conformance failure list.

conformance/failure_list_objc.txt

@@ -1,4 +1,6 @@
-# All tests currently passing.
-#
 # JSON input or output tests are skipped (in conformance_objc.m) as mobile
 # platforms don't support JSON wire format to avoid code bloat.
+Required.ProtobufInput.IllegalZeroFieldNum_Case_0
+Required.ProtobufInput.IllegalZeroFieldNum_Case_1
+Required.ProtobufInput.IllegalZeroFieldNum_Case_2
+Required.ProtobufInput.IllegalZeroFieldNum_Case_3