test that students are auto-assigned to cohorts in partition scheme

@andy-armstrong @cahrens 

JIRA: TNL-943
not here...here the default cohort is being lazily created (by asking for a listing) but the student is unassigned.  the check happens inside CohortedPartitionScheme.get_group_for_user, which calls to cohorts.get_cohort(user), and inside that function, the student is lazily assigned a cohort according to whatever configuration is in effect.