test_external_inspection flaky on x86-64 Ubuntu 24.04 on free-threading

[ambv]

With #133284 applied, test_async_global_awaited_by occasionally fails, reporting fewer than 1000 pending tasks. Example failure:
https://github.com/python/cpython/actions/runs/14813289395/job/41590700582

Additionally, the oldest tasks in such case are missing the awaited_by link to a task group context manager:
(2199028913760, 'Task-25', [[['echo_client_spam'], 2199050127392]])

A healthy task reports being under the task group context manager:
(2199056497056, 'Task-1997', [[['_aexit', '__aexit__', 'echo_client_spam'], 2199049603104]])

I looked at it on macOS ARM and Ubuntu ARM, but it does not reproduce despite re-running the test with -F. On GHA it happens often enough for us to notice on the PR.

[pablogsal]

Looks like this is indeed not a free threading problem after all: #133368