spec: rake task for non parallel_exclude tests

Change-Id: I45379aacc391b68003c7ff66239d175a2a362e9f
Reviewed-on: https://gerrit.instructure.com/28663
Reviewed-by: Bryan Madsen <bryan@instructure.com>
QA-Review: Bryan Madsen <bryan@instructure.com>
Product-Review: Bryan Madsen <bryan@instructure.com>
Tested-by: Shawn Meredith <shawn@instructure.com>