Deregister ephimeral models from ModelPermissions

Explictly delete the ephimeral models from ModelPermission
registry to avoid them being lookedup in a subsequent tests
where they don't exist.

Signed-off-by: Roberto Rosario <roberto.rosario.gonzalez@gmail.com>

mayan/apps/acls/classes.py

@@ -12,6 +12,12 @@ class ModelPermission(object):
     _inheritances = {}
     _registry = {}
 
+    @classmethod
+    def deregister(cls, model):
+        cls._registry.pop(model, None)
+        # TODO: Find method to revert the add_to_class('acls'...)
+        # delattr doesn't work.
+
     @classmethod
     def register(cls, model, permissions):
         from django.contrib.contenttypes.fields import GenericRelation