mediaCommentThumbnail spec fix

properly teardown sinon stub

fixes CNVS-10965

Change-Id: Id02b3d5923aba6671018bb9b6dec51a7e7711b81
Reviewed-on: https://gerrit.instructure.com/29622
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Simon Williams <simon@instructure.com>
Product-Review: Bryan Madsen <bryan@instructure.com>
QA-Review: Bryan Madsen <bryan@instructure.com>
Reviewed-by: Mike Nomitch <mnomitch@instructure.com>