CI: Run multi-threading test job with execution context

[straight-shoota]

This is a much simpler alternative to #16329 which requires no refactoring. We already have a dedicated test job for multi-threading. It just doesn't use execution contexts yet. Adding that already gives great test coverage for the new feature.
It only runs std_spec but that should be fine. I wouldn't expect much benefit from running compiler specs as well.

There's not much point in keeping tests for only -Dpreview_mt without -Dexecution_context because we're not expecting to continue supporting the multi-threading preview with the old scheduler model.

[ysbaddaden]

There's a huge difference: with preview_mt alone we start multiple threads/schedulers automatically, so fibers will run in parallel, while execution_context only starts a single thread/scheduler and fibers only run concurrently.

There's value in testing the concurrent case, but we also want to test parallelism, so there should be two jobs:

1. EC enabled and 1 worker (concurrent)
2. EC enabled and N workers (parallel)

The second case needs either support in spec (stdlib) or at least in a spec helper (not using .default_workers_count because it defaults to the number of cpus):

{% if Fiber.has_constant?(:ExecutionContext) %}
  count = ENV["CRYSTAL_WORKERS"]?.try(&.to_i?) || 1
  Fiber::ExecutionContext.current.resize(count)
{% end %}

[straight-shoota]

I think we need to add the EC resize to stdlib spec package so it can work in any spec not just one including some specific helper. I think that makes sense, because this behaviour is useful outside of stdlib specs. It practically only has an effect if CRYSTAL_WORKERS is set.
We might want to extract that as a separate change, though.

Apparently, Fiber::ExecutionContext can be defined, even without -Dexecution_context. But then it's missing .current: for example https://github.com/crystal-lang/crystal/actions/runs/19719679745/job/56499752283?pr=16339 😞
Looks like ExecutionContext is spilling somewhere when it's not used: GlobalQueue, Runnables, FiberCounter are defined, but no methods.

[ysbaddaden]

The namespace is leaking in std specs because of spec/std/fiber/execution_context/* spec files that always require GlobalQueue and Runnables.

[ysbaddaden]

There should be a distinct PR to resize the default execution context in spec, and maybe we should think of a CLI arg to set the number of workers, that could default to ENV["CRYSTAL_WORKERS"]? || 1.

[straight-shoota]

It would be really nice if we had a simple test to check whether we're using execution contexts.
Fiber.has_constant?(:ExecutionContext) could be good. But it's a bit fragile. Even if we remove the currently existing definitions, it could easily slip in again (we'd probably notice, but it's still annoying).
Perhaps we should just check flag?(:execution_context)? And eventually migrate to !flag?(:no-mt)?

[ysbaddaden]

Checking flag?(:execution_context) isn't forward safe, so general libraries should check for Fiber.has_constant?(:ExecutionContext) instead (they're unlikely to pull the namespace) but stdlib might not need to bother and we'll just fix the flag when we release the feature, so 👍

[straight-shoota]

Rebased on top of #16444