Unit test for import performance STUD-1994

Unit test for this recurring issue: https://openedx.atlassian.net/browse/STUD-1994

@dmitchell @cpennington Please review.
Here's the evidence that the test returns a different value on Jenkins (1 less call).  I assume it's because of the presence of a memcache.
https://jenkins.testeng.edx.org/job/edx-all-tests-auto-pr/12445/SHARD=1,TEST_SUITE=unit/