Port `test_rpc` off `run_in_actor`

Sole call-site: `run_in_actor(sleep_back_actor, ...)` ->
blocking `tractor.to_actor.run(..., an=n, ...)` (#477 removal).
The RPC-callback subactor is awaited in-caller instead of
reaped at nursery teardown; `name=`/`enable_modules=` map to
`to_actor.run()`'s same-named params, the rest to `**fn_kwargs`.

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

tests/test_rpc.py

@@ -111,8 +111,9 @@ def test_rpc_errors(
 
             actor = tractor.current_actor()
             assert actor.is_registrar
-            await n.run_in_actor(
+            await tractor.to_actor.run(
                 sleep_back_actor,
+                an=n,
                 actor_name=subactor_requests_to,
 
                 name='subactor',