spec: fix outside_transaction? for Mysql2Adapter

Change-Id: Ia77d2e2d2bce42450e7936b1ae3f7feb6dcbd542
Reviewed-on: https://gerrit.instructure.com/28575
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Nick Cloward <ncloward@instructure.com>
Reviewed-by: Jacob Fugal <jacob@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>