ci: avoid races in tests asserting warnings

[davidhewitt]

This PR avoids races in tests which use warnings.capture_warnings.

While we had issues with Py_GIL_DISABLED with these tests previously, warnings.capture_warnings, not being thread-safe of course means that no two rust tests doing such work can be run concurrently, because even GIL switches can lead to tests interleaving and therefore getting bad results.

While we'd not observed these races on the GIL-enabled builds before, I think #5341 has actually gotten stuck due to this behavior because disallowing races to build types has introduced a new synchronization point between test threads.

I fix that problem in this PR by introducing a mutex into the CatchWarnings test helper; as long as all tests go through that helper they will be forced to run sequentially for the sensitive block. I had to rearrange the "test utils" file a bit so that it was available at crate::test_utils in all places it's used, so that the macros inside it work properly.

As a result, I think I can enable all these tests on the free-threaded build too :)

cc @ngoldbaum

[davidhewitt]

CI seems fully green, I'm going to merge this as a tests-only change. Please forgive 🙏

[ngoldbaum]

Cool, I'm glad you were able to re-enable a bunch of tests!

[ngoldbaum]

By the way, Python 3.14 and newer has thread-safe warnings. It's turned on by default on the free-threaded build but you need to enable it with a new -X CLI option on the GIL-enabled build. See https://docs.python.org/3.14/whatsnew/3.14.html#concurrent-safe-warnings-control.