Add GPU tests with buildkite

[devmotion]

Since XGBoost claims to be GPU- (or rather CUDA-) compatible, I think GPU tests should be part of the CI tests. It seems currently GPU support is not tested in CI and has to be checked locally on a system with NVIDIA GPUs. This could be changed by setting up buildkite (https://github.com/JuliaGPU/buildkite). It's very straightforward to get it up and running.