Fix assertion in test_make_stats_for_no_group

Tests were failing on Travis-CI with:

AttributeError: 'function' object has no attribute 'assert_called_once'

In 9bd4160, we added a unit test for make_stats_for_no_group,
with an assertion using assert_called_once. However, this was
only introduced in Python 3.6, and tests run on 3.5.

Change the test to use assert_called_once_with.

(Note how autospeccing saved the day here)