Add multithreaded test scenarios to start testing free-threading

[lysnikolaou]

This adds two tests that:

• Dump and parse an actual YAML file that sort of looks like a Github Actions file. This is done in 8 threads for 200 files in parallel. ~10% of the files are invalid so that erroneous files are tested as well.
• Spawn 8 threads after having registered a constructor and a resolver. This tests those registries as well. In each thread, a loop with 200 iterations is ran, where in each iteration a random YAML code sample is dumped and loaded. Input and output are then compared to one another.

[nitzmahone]

Thanks- running these manually was helpful in gaining some confidence that "normal" usages of PyYAML under a free-threaded build were reasonably safe (hence the reason I've added the free-threaded support compiler directive to the extension for main and 6.0.3). I'll try to figure out the best place to get these tests plugged into the regular CI process when I switch everything around to support abi3 and the (hopefully) upcoming abi3.abi3t builds for 3.15+.

[lysnikolaou]

Sounds good! Feel free to ping me in case I can help out in any way when you start working on support for abi3t.