Bug 1011274 - increase mocha timeout when loading blobs r=azasypkin

Just curious can we just get rid of these "real" images and use technique similar to https://github.com/azasypkin/gaia/blob/d4583df788d33119f1fc4c68dca40a986a9bf18b/apps/sms/test/unit/utils_test.js#L1118? It think it should a way faster that XHR.