Clarify date stamp on 'screwed students' CSV filename
Add testStats function to LSRAssignmentTest

This sanity-checks the basic stats and also evaluates the 'student
screwedness' metric referenced by Shulin's CSV generator.
Tweak lottery assignment stats test

We were getting incorrect results for the student screwedness value in
some cases.  This was because some students were marked as Interested in
some of the same classes marked as Priority/1.  While we discourage this,
it is possible that some real students will encounter this situation so I
think the test should accommodate it.
Add docstring for LSRAssignmentTest.testStats()