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

We need to make sure this test is stable. The test is failed on my local.
The error message:
```
  1) airplaneMode settings Turning airplaneMode from on to off all related services will be turned on:
     
  AssertionError: false == true
      at Context.<anonymous> (/Users/itoyxd/evan/dev/mozilla/gaia/apps/settings/test/marionette/tests/airplane_mode_settings_test.js:47:14)
      at callFn (/Users/itoyxd/evan/dev/mozilla/gaia/node_modules/mocha/lib/runnable.js:223:21)
      at Test.Runnable.run (/Users/itoyxd/evan/dev/mozilla/gaia/node_modules/mocha/lib/runnable.js:216:7)
      at Runner.runTest (/Users/itoyxd/evan/dev/mozilla/gaia/node_modules/mocha/lib/runner.js:373:10)
      at /Users/itoyxd/evan/dev/mozilla/gaia/node_modules/mocha/lib/runner.js:451:12
      at next (/Users/itoyxd/evan/dev/mozilla/gaia/node_modules/mocha/lib/runner.js:298:14)
      at /Users/itoyxd/evan/dev/mozilla/gaia/node_modules/mocha/lib/runner.js:308:7
      at next (/Users/itoyxd/evan/dev/mozilla/gaia/node_modules/mocha/lib/runner.js:246:23)
      at /Users/itoyxd/evan/dev/mozilla/gaia/node_modules/mocha/lib/runner.js:270:7
      at done (/Users/itoyxd/evan/dev/mozilla/gaia/node_modules/mocha/lib/runnable.js:185:5)
      at callFn (/Users/itoyxd/evan/dev/mozilla/gaia/node_modules/mocha/lib/runnable.js:228:7)
      at Hook.Runnable.run (/Users/itoyxd/evan/dev/mozilla/gaia/node_modules/mocha/lib/runnable.js:216:7)
      at next (/Users/itoyxd/evan/dev/mozilla/gaia/node_modules/mocha/lib/runner.js:258:10)
      at Object._onImmediate (/Users/itoyxd/evan/dev/mozilla/gaia/node_modules/mocha/lib/runner.js:275:5)
      at processImmediate [as _immediateCallback] (timers.js:330:15)
```