Bug 1727820 - Add testing for `AppUpdater.stop` r=bhearsum

`AppUpdater.stop()` is not currently well tested, and the new implementation relies on throwing exceptions. It's important that these exceptions be properly caught to ensure that we don't end up handling the exception as an internal error or not handling it properly at all.

Differential Revision: https://phabricator.services.mozilla.com/D159308