Use defaultUser.png instead of defaultUser.gif

Indeed, defaultUser.gif is in a deprecated skins folder (Products/CMFPlone/
skins/plone_deprecated/defaultUser.gif) and not defaultUser.png
(Products/CMFPlone/skins/plone_images/defaultUser.png).
Test is also updated.

plone/app/discussion/browser/comments.py

@@ -411,7 +411,7 @@ class CommentsViewlet(ViewletBase):
 
         if username is None:
             # return the default user image if no username is given
-            return 'defaultUser.gif'
+            return 'defaultUser.png'
         else:
             portal_membership = getToolByName(self.context,
                                               'portal_membership',