`test_worker_thread_context_not_leaked` sometimes fails

cc @EmmanuelNiyonshuti I haven't dug into this yet but the test is flakey. I'm opening this instead of simply commenting in the flakey-tests thread so we can discuss! See https://github.com/A5rocks/trio/actions/runs/30208152029 and https://github.com/A5rocks/trio/actions/runs/29643389046