Bug 1024395 - Lockscreen color overlay not working due to bug 973443

Please refer to https://github.com/mozilla-b2g/gaia/pull/19712/files#diff-39d8fcc7bf1d607a5a5b99794f1d7b69
nit: Let's use `sinon.assert.calledWith(wallpaper._setWallpaperColor, testColor);` as it is more readable.