Add anti-hang `fail_after` cap to aio-cancel test

Wrap `test_tractor_cancels_aio`'s `main()` in a
`trio.fail_after(9 * cpu_perf_headroom())` so a wedged remote
runtime can't hang the test forever. This is the blessed
anti-hang guard here bc `pytest-timeout`'s global cap is
intentionally off (it breaks `trio` under the fork backends,
per the `pyproject` NOTE).

The cap is generous + CPU-headroom-scaled bc it's an anti-hang
guard, not a perf assertion. Motivated by the
`._ria_nursery`-removal regression where a wedged ria-reaper
once hung this exact test indefinitely.

(this patch was generated in some part by [`claude-code`][claude-code-gh])
[claude-code-gh]: https://github.com/anthropics/claude-code

tests/test_infected_asyncio.py

@@ -207,6 +207,15 @@ def test_tractor_cancels_aio(
 
     '''
     async def main():
+        # anti-hang wall-clock cap: a per-test `trio.fail_after`
+        # is the blessed guard here since `pytest-timeout`'s
+        # global cap is intentionally off (see the `pyproject`
+        # NOTE — it breaks trio under fork backends). Generous +
+        # CPU-headroom-scaled bc this is an anti-hang guard, not
+        # a perf assertion; a wedged ria-reaper once hung this
+        # test forever (the `._ria_nursery`-removal regression).
+        from .conftest import cpu_perf_headroom
+        with trio.fail_after(9 * cpu_perf_headroom()):
             async with tractor.open_nursery(
                 debug_mode=debug_mode,
                 registry_addrs=[reg_addr],