Bug 981560 - [settings] add marionette test for device information 

fixes Bug 981560 - [settings] add marionette test for device information - More information panel 
Sorry, you might misunderstand.

You might found out DeveloperMenuShowed is actually opened by default and it make the test meaningless.
So, I suggest 
1. rename this function as clickDeveloperMenuCheckBox and remove the conditions.
2. store value of isDeveloperMenuVisible before test and it should be different after we click the checkbox.