Port `test_runtime` off `run_in_actor`

Sole call-site: an inlined `run_in_actor(...).result()` ->
blocking `tractor.to_actor.run(fn, an=an, ...)` (#477 removal).
Behaviour identical — the one-shot's result/error is awaited
in the caller's task rather than reaped at nursery teardown;
the enclosing `move_on_after` still cancels the sub in the
`error_in_child=False` case.

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

tests/test_runtime.py

@@ -78,13 +78,12 @@ async def test_lifetime_stack_wipes_tmpfile(
             async with tractor.open_nursery(
                 loglevel=loglevel,
             ) as an:
-                await (  # inlined `tractor.Portal`
-                    await an.run_in_actor(
+                await tractor.to_actor.run(
                     crash_and_clean_tmpdir,
+                    an=an,
                     tmp_file_path=path,
                     error=error_in_child,
                 )
-                ).result()
     except (
         tractor.RemoteActorError,
         BaseExceptionGroup,