added week 5 reading quiz; cleaned up assignment linking

Made assign-test-79-s13 point to s79-s13-assign.json instead of
test79-s13-assign.json to eliminate the need to edit the latter.  The
only difference now is the due date files. Since the due dates control
whether an assignment even shows up, the assignments being tested still
are not displayed to the students until s79-s13-due.json is modified to
include the assignment.