Bug 1073157 - [System] fix wallpaper_manager.js tests with a newer mocha...

... r=etienne
Yes, that's implemented for ages in https://github.com/mozilla-b2g/gaia/blob/c4657a4a69b937cc289e7b035a9499a1772cb344/dev_apps/test-agent/common/test/mocha_generators.js#L36-L49 (I think @lightsofapollo did it before I even joined Mozilla).

I also learnt about this by chance (looking at some tests made by James), I agree this could be more publicized... Possibly even upstreaming it to mocha !

The failing test was happening here so I did this change to see more clearly what happened instead of a useles timeout, like you correctly guessed :)