Use lib directory instead of package root for stacktrace checking

Author: fongandrew

lib/test.js

@@ -216,7 +216,7 @@ Test.prototype._assert = function assert (ok, opts) {
     if (!ok) {
         var e = new Error('exception');
         var err = (e.stack || '').split('\n');
-        var dir = path.dirname(__dirname) + path.sep;
+        var dir = __dirname + path.sep;
 
         for (var i = 0; i < err.length; i++) {
             var m = /^[^\s]*\s*\bat\s+(.+)/.exec(err[i]);