Unit test for import performance STUD-1994

Unit test for this recurring issue: https://openedx.atlassian.net/browse/STUD-1994

@dmitchell @cpennington Please review.
@cpennington I've added a new `check_exact_number_of_calls` method in addition to this one.  I have to keep this one since the number of calls on Jenkins, which has memcache, is different than locally.