Future: use @testsetup to reduce time to run tests on GPU

[rkierulf]

Feature Request

For the Buildkite CI set up in #411 , we are currently calling Pkg.add("") in each test item for whichever GPU backend we are testing. This is to avoid having all four backends as a dependency in test/Project.toml, and the Pkg.add() step must be done in each test because of the way TestItemRunner.jl runs test items in separate modules.

However, calling Pkg.add() each time a test item is run introduces overhead that could possibly be avoided. It looks like there is a @testsetup macro under development (julia-vscode/TestItemRunner.jl#71) which would allow only adding the packages once, so we should look into using this once it is complete.

[rkierulf]

Update: The previous approach of calling Pkg.add() in the middle of the tests led to some version dependency issues. We are now doing something closer to DiffEqGPUs approach here: https://github.com/SciML/DiffEqGPU.jl/blob/master/.buildkite/runtests.yml, so the overhead mentioned above with running the tests is no longer an issue.