fixes in unit tests

tests/migration/__init__.py

@@ -1,8 +1,11 @@
+import unittest
+
 from convert_to_new_inheritance_model import *
 from decimalfield_as_float import *
-from refrencefield_dbref_to_object_id import *
+from referencefield_dbref_to_object_id import *
 from turn_off_inheritance import *
 from uuidfield_to_binary import *
 
+
 if __name__ == '__main__':
     unittest.main()