test_shielded_cancel_sleep_time is unstable on slow build machines and slow platforms (e.g., ppc64le)

[mcepl]

Things to check first

• I have searched the existing issues and didn't find my bug already reported there

• I have checked that my bug is still present in the latest release

AnyIO version

4.14.2

Python version

3.14.6

What happened?

When packaging for openSUSE/Factory I got this persistent failure of test_shielded_cancel_sleep_time:

___________________ test_shielded_cancel_sleep_time[asyncio] ___________________
[  554s]   + Exception Group Traceback (most recent call last):
[  554s]   |   File "/home/abuild/rpmbuild/BUILD/python-anyio-test-4.14.2-build/anyio-4.14.2/tests/test_taskgroups.py", line 1450, in test_shielded_cancel_sleep_time
[  554s]   |     async with create_task_group() as tg:
[  554s]   |                ~~~~~~~~~~~~~~~~~^^
[  554s]   |   File "/usr/lib/python3.13/site-packages/anyio/_backends/_asyncio.py", line 815, in __aexit__
[  554s]   |     raise BaseExceptionGroup(
[  554s]   |         "unhandled errors in a TaskGroup", self._exceptions
[  554s]   |     ) from None
[  554s]   | ExceptionGroup: unhandled errors in a TaskGroup (1 sub-exception)
[  554s]   +-+---------------- 1 ----------------
[  554s]     | Traceback (most recent call last):
[  554s]     |   File "/home/abuild/rpmbuild/BUILD/python-anyio-test-4.14.2-build/anyio-4.14.2/tests/test_taskgroups.py", line 1458, in test_shielded_cancel_sleep_time
[  554s]     |     assert (time.process_time() - process_time) < hang_time
[  554s]     | AssertionError: assert (183.655833728 - 183.448973366) < 0.2
[  554s]     |  +  where 183.655833728 = <built-in function process_time>()
[  554s]     |  +    where <built-in function process_time> = time.process_time
[  554s]     +------------------------------------

How can we reproduce the bug?

Running the test suite on overloaded build system (OpenSUSE OpenBuild system in my case) with slow processor (ppc64le in my case)