Bug 995687 - Move DUMP overwriting to suiteSetup and suiteTeardown

https://bugzilla.mozilla.org/show_bug.cgi?id=995687
yeah this is likely better. However, if the mock was called MockDUMP, we could simply use the MocksHelper to manage this easily. Maybe more involved than what you want to do though.

Also if tests are failing because of this, it's also likely that they have asynchronous behavior and they don't properly wait until they end. As a result, they can have other subtle racing issues... That's why I'd like to see some of the actual failures you currently see :)