Bug 1062829 - Converted Json methods from XHR to LazyLoader in the FTU A...

...PP

https://bugzilla.mozilla.org/show_bug.cgi?id=1062829
I think that using a stub that returns Promise.resolve() is the right way to go because it makes the test fully synchronous. I'd do it across all tests so that they become fully synchronous (at least for the part of loading the configuration).