Bug 1081065 - Fix broken test coverage

No matter the order between box.mocha.run and box.dispatchEvent, we just make sure that we take custom reporter in blanket.js at [1] so that prevent blanket to launch after window.onload. Therefore, emitted 'mocha-run' to tell blanket start to work.

[1] https://github.com/mozilla-b2g/gaia/pull/25286/files#diff-a7d99a2a2a12a89ba6ea7dcfcc6fc05cR67