The old name no longer makes sense, since it no longer exports an spawn_task
symbol, and add_task is what we really care about.
MozReview-Commit-ID: IE7B8Czv8DH
--HG--
rename : testing/mochitest/tests/SimpleTest/SpawnTask.js => testing/mochitest/tests/SimpleTest/AddTask.js
extra : rebase_source : 03bca5aa69a7625a49b4455a6c96ce4c59de3a5a