Convert canvas_stringex to use rspec2

Fixes CNVS-11915

No functional changes, only changes to test files.

Change-Id: I0880243af3ce971b44074d6d7afc4099287d720c
Reviewed-on: https://gerrit.instructure.com/32513
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Bryan Madsen <bryan@instructure.com>
QA-Review: Bryan Madsen <bryan@instructure.com>
Reviewed-by: Simon Williams <simon@instructure.com>