Unit test for import performance STUD-1994

Unit test for this recurring issue: https://openedx.atlassian.net/browse/STUD-1994

@dmitchell @cpennington Please review.
I might go further and actually just say that we expect the calls to be exact, rather than just less than the supplied value. That will catch issues like this, and also force us to update the peak if we do actually reduce the number of calls.