Use a better strict mode detect for tests.js so older versions of Node.js pass.

Former-commit-id: fd9f6ea35c71a8183ce0dcf4a6ec6e6afe13c39e

test/test.js

@@ -1706,7 +1706,7 @@
       var thisArg,
           array = ['a'],
           callback = function() { thisArg = this; },
-          useStrict = Function('"use strict";return this')() === undefined;
+          useStrict = (function() { return this; }).call(null) === null;
 
       var funcs = [
         'countBy',