make grade passback specs more robust

especially in rails 3

just use stubs instead of starting up a second thread and opening a
connection

Change-Id: I52837c6ff65ed62a225c0a4df61c1a73df2a1526
Reviewed-on: https://gerrit.instructure.com/31320
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: James Williams  <jamesw@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>