Make test faster and more specific
Make test faster and more specific

This test was using the time taken to pass as a proxy for whether it was
green or not (with the idea that a very long-running test would annoy
someone enough to consider it failing).

This commit:
- makes the test run quicker by monkey-patching `max_rows` to a much
  smaller value
- add some mocks to test for the behaviour that we are looking for, i.e.
  that expensive functions are not being called unnecessarily when the
  CSV file is bigger than our limit

alphagov/notifications-utils