Bug 1078212 - Use the shared mock_l10n in Homescreen

https://bugzilla.mozilla.org/show_bug.cgi?id=1078212
I Indeed should keep realL10n like in request.js test at the beginning when pre-mocking it and then reset it in teardown setup. Fixing it thanks !

GridManager (app/js/grid.js) is an anonymous function that returns an object. It requires to have navigator.mozL10n.get preinitialized to return a valid object like app/js/request.js.

If I don't pre-mock. When I get in setup the function will get called and mozl10n will be undefined so GridManager will return undefined and won't be able to use it for test.