Fix false positive in test due to changed error

Under Python 3, the error message that is expected to be returned has
changed. This fixes the 'expected' error so that the test succeeds and
fails as required.

main/tests/test_donor_import.py

@@ -44,7 +44,7 @@ def test_decode_subject(self):
     def test_invalid_args(self):
         with self.assertRaises(CommandError) as e:
             call_command('donor_import')
-        self.assertIn('Error: too few arguments', str(e.exception))
+        self.assertIn('Error: the following arguments are required', str(e.exception))
 
     def test_invalid_path(self):
         with self.assertRaises(CommandError) as e: