feat(tests): Add helper to count mock calls

[lobsterkatie]

The Python built-in MagicMock class has assert_called_once_with, assert_any_call, assert_has_calls, etc, but none of those is helpful if what you really want is assert_called_twice_with or assert_called_exactly_three_times_with. This therefore adds a new testing helper, count_matching_calls, which works exactly the same way as the built-in ones, except that you also have to pass it the mock. So, for example, the following sets of assertions are equivalent (yes, I know some of the mock methods don't actually exist - that's the point):

some_mock.assert_called_once_with("dogs", "are", "great", adopt_dont_shop=True)
assert count_matching_calls(some_mock, "dogs", "are", "great", adopt_dont_shop=True) == 1

some_mock.assert_called_twice_with("dogs", "are", "great", adopt_dont_shop=True)
assert count_matching_calls(some_mock, "dogs", "are", "great", adopt_dont_shop=True) == 2

some_mock.assert_called_exactly_three_times_with("dogs", "are", "great", adopt_dont_shop=True)
assert count_matching_calls(some_mock, "dogs", "are", "great", adopt_dont_shop=True) == 3

This will be helpful for testing an upcoming PR involving caching, where for example it will be nice to be able to test that cache.get is called twice while cache.set is only called once (or not at all, if certain criteria aren't met).

[cvxluo]

nice change