Add a DEP-8 test checking that Zenity text input works

I am happy with this test, I just have one reservation.

• Should it be marked superficial? Since Zenity is a very simple program and this is core functionality, I think not. A simple Xdotool test such as this would be superficial for most programs, but since it's really all Zenity does is take the input and give it to the caller, I think it's appropriate.