Use Base.Semaphore to control test execution parallelism

[giordano]

This is a refactoring of the code, to then enable #77 in a follow up PR (CC @christiangnrd).

The idea of using a semaphore is mine, initial implementation is from Claude, but I made a few manual refinements afterwards (some of them folded in the first commit). Overall summary of the changes:

Replace the fixed worker-task-per-slot model with a semaphore-based
approach: one task per test, with a Base.Semaphore(jobs) limiting
concurrency and a Channel-based worker pool for reuse. This decouples
the number of tasks from the parallelism level and simplifies the
control flow (no inner while loop, tests array is immutable).

I'm mostly happy about the result, it's very close to what I had in mind, and the net diff is relatively small (+48/-22), so this shouldn't be too hard to review.

#118 was useful because it detected that the case of no tests to run wasn't handled correctly, so yay for the extra tests.

[giordano]

The core change I was interested in was changing this for loop from an iteration over the workers, to an iteration over the tests: the idea for #77 is to have different semaphores for different subsets of tests, and here we iterate over the different subsets (and associated semaphores). The whole execution cycle would then require only this change on this single line, the rest would remain as is.

I still need to fully work out the user-facing changes for designating the serial tests for #77. I have some bad ideas in mind, but I want to decouple them from this refactoring, which I believe is still worth on its own right for making the code easier to follow.

[giordano]

Uhm, the test failure

default workers stopped at end: Test Failed at /Users/runner/work/ParallelTestRunner.jl/ParallelTestRunner.jl/test/runtests.jl:472
  Expression: after == before
   Evaluated: 1 == 2
  Stacktrace:
   [1] top-level scope
     @ ~/work/ParallelTestRunner.jl/ParallelTestRunner.jl/test/runtests.jl:10
   [2] macro expansion
     @ ~/hostedtoolcache/julia/nightly/aarch64/share/julia/stdlib/v1.14/Test/src/Test.jl:2243 [inlined]
   [3] macro expansion
     @ ~/work/ParallelTestRunner.jl/ParallelTestRunner.jl/test/runtests.jl:424 [inlined]
   [4] macro expansion
     @ ~/hostedtoolcache/julia/nightly/aarch64/share/julia/stdlib/v1.14/Test/src/Test.jl:2243 [inlined]
   [5] macro expansion
     @ ~/work/ParallelTestRunner.jl/ParallelTestRunner.jl/test/runtests.jl:472 [inlined]
   [6] macro expansion
     @ ~/hostedtoolcache/julia/nightly/aarch64/share/julia/stdlib/v1.14/Test/src/Test.jl:781 [inlined]

is interesting:

1. it happened only on macOS with Julia nightly, but that's precisely what I use locally, and tests passed for me several times 😅
2. the fact that there were fewer subprocesses still alive after the tests finished than before is quite surprising: if something went wrong with the termination of the subprocesses I'd expect more processes to be still around, not less.

The error disappeared after a rerun, I'd say it was a glitch? 😬 Edit: tests on all platforms always passed in the following 3 full reruns.