[#112] Added tests for adding/deleting groups and skills. Also added extra locators and methods relating to groups and skills

This is my first time contributing to Mozilla, so please be liberal with the criticisms!
I added a few tests for checking whether a user can add or remove groups as per #112 . I also did the same for skills. I added a few locators and methods related to adding and deleting groups/skills in order to do these tests, though I haven't added checking for whether the delete button icon is a valid image or not.