spec: fix attachment_spec timecop inteference with other specs

Change-Id: I57524aa4b15e49ea2742332d55713235272c6f10
Reviewed-on: https://gerrit.instructure.com/34132
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Shawn Meredith <shawn@instructure.com>
Product-Review: Shawn Meredith <shawn@instructure.com>
QA-Review: Shawn Meredith <shawn@instructure.com>