Rescale execution context in spec runner with CRYSTAL_WORKERS

[straight-shoota]

When running specs with execution contexts, it's annoying to have to explicitly scale the default execution context in order to get actual parallelism. This could be handled in some kind of spec helper, but it would still require extra effort and it'd be easy to miss out on it.

Thus, I'm suggesting to resize the EC in the stdlib spec package (i.e. for any spec program) if the environment variable CRYSTAL_WORKERS is defined.

Extracted from #16339

[ysbaddaden]

NOTE: the reason we don't handle CRYSTAL_WORKERS with execution contexts by default is for applications to opt-in to MT if they want to, when they want to (so they can initialize single-threaded, or prefer to start explicit contexts) and to use their own controls like CLI argument, custom ENV variable, file configuration, ...

A rogue CRYSTAL_WORKERS in the environment shouldn't affect an applications' behavior.

And... now I'm having second thoughts about using it in spec 😅

I guess unlike prelude that's always required and executed before applications can do anything, you can require spec after requiring your app.

[straight-shoota]

I suppose we could put this behind another feature flag which keeps being opt-in even when -Dexecution_context becomes the default. Maybe that makes sense in general...?
An -Dmt flag which automatically resizes the default context?
Let's keep thinking about that. But I think this is fine for now as we try to set up CI for execution contexts.

[ysbaddaden]

Yes, I believe it's fine for spec 👍