spec: use truncate_all_tables helper

Change-Id: I52ceb101adb933e42ad98230ac034a0f5e869156
Reviewed-on: https://gerrit.instructure.com/31739
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>