Parallelise tests in build

[st--]

Currently the rest of the build takes ~40 minutes anyways so that shouldn't make it longer overall.

IMO this is a motivation for splitting the tests into different groups that are run in parallel (similar what we do in the SciML ecosystem: https://github.com/SciML/OrdinaryDiffEq.jl/blob/master/test/runtests.jl) rather than running the examples in serial.

Originally posted by @devmotion in #303 (comment)

[david-vicente]

Related to but a different issue is not being able to run sections of the tests separately through the command line. I've been thinking of implementing the code to allow the use of a Pkg.Test feature that was added for this purpose. I think that potential contributors will have an easier time when experimenting stuff if this gets added.

[david-vicente]

Maybe both ideas can be implemented at the same time?

[devmotion]

This is already part of my suggestion above. You would just set e.g. ENV["GROUP"] = "BaseKernels".

[david-vicente]

Oh that also works with command line arguments?

[devmotion]

There are no command line arguments involved here. It is based on an environment variable that you can set in your shell before starting Julia or in the Julia REPL with ENV[...] = ... or withenv(... => ...) do .....

[david-vicente]

Understood. So it is a different approach that in a way includes my suggestion but has more functionality.