Make possible test Security for Anonymous

 With this change, by using "None" instead username at any SecurityTestCase
 assertion, it test security as Anonymous. ie.:
   self.failIfUserCanViewDocument(None, document)

product/ERP5Type/tests/SecurityTestCase.py

@@ -33,6 +33,7 @@ from pprint import pformat
 from AccessControl.SecurityManagement import newSecurityManager
 from AccessControl.SecurityManagement import getSecurityManager
 from AccessControl.SecurityManagement import setSecurityManager
+from AccessControl import SpecialUsers
 from Products.DCWorkflow.Transitions import TRIGGER_USER_ACTION
 
 from Products.ERP5Type.tests.ERP5TypeTestCase import ERP5TypeTestCase
@@ -128,7 +129,11 @@ class SecurityTestCase(ERP5TypeTestCase):
 
   def _loginAsUser(self, username):
     """Login as a given username. The user must exist.
+       In case Username is None, we consider test as Anonymous.
     """
+    if username is None:
+      newSecurityManager(None, SpecialUsers.nobody)
+    else:
       uf = self.getPortal().acl_users
       user = uf.getUserById(username)
       self.assertNotEquals(user, None, 'No user %s' % username)