Ensure consistent calculation of duration

probably doesn't make much of a difference, but seems more correct

Author: FND

lib/reporters/base.js

@@ -304,7 +304,7 @@ function Base (runner) {
 
   runner.on('end', function () {
     stats.end = new Date();
-    stats.duration = new Date() - stats.start;
+    stats.duration = stats.end - stats.start;
   });
 
   runner.on('pending', function () {