[feat] Add UI capabilities to CI

In order to make QtTesting work on unix based runners, the CI needs UI capabilities; a common method involves xhost display hikacking. (It is also possible to run UI apps within docker if we ever get there).

Qt Tests successfully ran remotely on Brave as gitlab-runner under these conditions:

export DISPLAY=:0.0; xhost + local: #Needs to be invoked as Kitware user, having proper XAUTHORITY
usermod -a -G video gitlab-runner
DISPLAY=:0 ctest -R TestQt -VV

Not tested on OSX "Mini", but workflow should be the same.

Adding DISPLAY=:0 to the environment in the CI script would make it feasible, but I am unsure if setting this variable would be scoped to the .test section ?

@nick.laurenson can you check the syntax / advise on better scoping methods ?