Add xfail to test_ki_protection_doesnt_leave_cyclic_garbage on Python 3.14

Fixes #3209; works around python/cpython#125603.

Author: musicinmybrain

src/trio/_core/_tests/test_run.py

@@ -2817,6 +2817,11 @@ def no_other_refs() -> list[object]:
     sys.implementation.name != "cpython",
     reason="Only makes sense with refcounting GC",
 )
+@pytest.mark.xfail(
+    sys.version_info >= (3, 14),
+    reason="https://github.com/python/cpython/issues/125603",
+    strict=True,
+)
 async def test_ki_protection_doesnt_leave_cyclic_garbage() -> None:
     class MyException(Exception):
         pass