spec: create announcements in the past to avoid sub-second precision problems in mysql

refs CNVS-7032

Change-Id: Iccb42f6f2ae2a13f2a3c002305c76ceb405693e0
Reviewed-on: https://gerrit.instructure.com/28803
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Brian Palmer <brianp@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>