Bug 968619 - [Settings] add marionette test for airplane mode

Could we have the `enableAirPlaneMode` method in ` apps/settings/test/marionette/app/regions/airplane_mode.js`? Then we could use `enableAirPlaneMode()`/ `enableAirPlaneMode(false)` to enable/disable air plane mode. And we don't need any `client.waitFor` logic here.

For example, we could do
```
airplaneMode.enableAirPlaneMode(false);
assert.equal(airplaneMode.wifiDesc.text(), 'Disabled');
...
```