generalize overly specific parsing test

We shouldn't be testing to see that a parse specifically returns
Nil, since in the future it will be returning Failure.  In any
case, that's not what this test is trying to test, so it shouldn't.

Author: TimToady

S05-grammar/example.t

@@ -88,7 +88,7 @@ is($content ~~ m/<schedule>/, $content, 'match rule');
         token b { b }
         token TOP { <a><b><before $<b>> }
     }
-    is rt81136.parse('abc'), Nil,
+    nok rt81136.parse('abc'),
         'lookbehind that should not match does not match';
 }