Multi-threaded execution

[davidanthoff]

We should add the ability to run each @testitem in a new task and schedule them on multiple threads.

[nsajko]

in a new task

In that case, it may also make sense to allow setting a stack size for the Task. And set it to a low value by default, to catch potential stack overflows:

PR that added relevant documentation:

• Document Task(f, n) API JuliaLang/julia#55184

The default stack size is system-dependent, so I think the default stack size for a test could be the minimum of the system-specific sizes. Or, e.g., half of the minimum.

• https://github.com/JuliaLang/julia/blob/ead23dbe1032d96476722a3d6d62c38a7f17f18e/src/options.h#L100-L117