spec: tweak single thread sel spec rake

Change-Id: Ia7c9a7eb2219b04dcbea2650a168d42446e9ae3f
Reviewed-on: https://gerrit.instructure.com/44070
Reviewed-by: Bryan Madsen <bryan@instructure.com>
Product-Review: Bryan Madsen <bryan@instructure.com>
QA-Review: Bryan Madsen <bryan@instructure.com>
Tested-by: Bryan Madsen <bryan@instructure.com>