Split teacher_scheduler_spec.rb in two

Some specs were moved to the new file, and boilerplate was brought
along with them. I removed a disused method
Other than that, no changes.

Refs CNVS-15688

Test Plan:

Verify that on Jenkins, neither file takes longer than two minutes to
execute.

Change-Id: Ib36211f3d0306a5d0668832f88209c1a67577630
Reviewed-on: https://gerrit.instructure.com/41483
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Joel Hough <joel@instructure.com>
Reviewed-by: Brad Horrocks <bhorrocks@instructure.com>
Product-Review: Matthew Wheeler <mwheeler@instructure.com>
QA-Review: Matthew Wheeler <mwheeler@instructure.com>