Merge AccessControl.User.tests.testUser fixes from 2.12, w/ LP#142536 fix.

doc/CHANGES.rst

@@ -155,6 +155,8 @@ Features Added
 Bugs Fixed
 ++++++++++
 
+- LP #142563:  Fix ``AccessControl.User.NullUnrestrictedUserTests.__str__``.
+
 - LP #267820:  Fix bad except clause in the ``sequence_sort`` method of
   the ``<dtml-in>`` tag.
 

src/AccessControl/User.py

@@ -400,6 +400,9 @@ class NullUnrestrictedUser(SpecialUser):
     def has_permission(self, permission, object):
         return 0
 
+    def __str__(self):
+        # See https://bugs.launchpad.net/zope2/+bug/142563
+        return repr(self)
 
 
 def readUserAccessFile(filename):