Fix unbound `timeout` under non-trio/MTF backends

`test_nested_multierrors`'s backend/depth budget `match` only
carries arms for the `trio` + `main_thread_forkserver` spawn
backends, so running under any other (e.g. `mp_spawn`) leaves
`timeout` unbound and crashes with an `UnboundLocalError` at the
headroom-scaling below. Add default per-depth arms riding the MTF
budgets (same per-spawn round-trip cost class).

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

tests/test_cancellation.py

@@ -647,6 +647,14 @@ async def test_nested_multierrors(
             timeout = 16
         case ('main_thread_forkserver', 3):
             timeout = 30
+        # any other fork-based backend (`mp_spawn` et al) pays
+        # the same per-spawn round-trip costs as MTF so rides
+        # its budgets; without a default arm `timeout` is left
+        # unbound -> `UnboundLocalError` at the scaling below.
+        case (_, 1):
+            timeout = 16
+        case (_, 3):
+            timeout = 30
 
     # inflate the budget by the throttle headroom probed above so
     # a slow box doesn't masquerade as a deadline regression.