CI for testing GPU functionality

[ArnoStrouwen]

The GPU functionality of this package is currently not tested, and from a quick local test seems to have regressed.
This package should be set up with buildkite.

[ChrisRackauckas]

I setup the pipeline: https://buildkite.com/julialang/highdimpde-dot-jl but we need to setup the CI.

[ChrisRackauckas]

The tests should also split to safetestsets and split into test groupings, core and GPU