[lldb][NFC] Add basic test for GUI command

Summary:
This adds a basic test for the GUI command. Just tests that it starts up, that we can quit the gui
and help window, and that the basic UI elements are rendered. Mostly testing the waters how
testing this command will do on the bots or if that will cause some serious issues when we do
fancy ncurses stuff.

Reviewers: labath, clayborg

Reviewed By: labath

Subscribers: JDevlieghere, lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D67018

llvm-svn: 370625