Unit test for import performance STUD-1994

Unit test for this recurring issue: https://openedx.atlassian.net/browse/STUD-1994
**Note:** There was a bug in this test since it didn't actually count the calls.  The number of calls was always 0, which was of course less than the expected maximum.  I have now introduced an expected minimum (defaulted to 1) to prevent this issue in the future.