: python/tests/test_actors.py: use context mgr for FD +sys.stdout/sys.stderr redirecction (#1950)

Summary:

do the stdio redirection with a context manager too (promised follow-up in the preceding diff)
```
  async def test_example() -> None:
      with configured(
          enable_log_forwarding=True, enable_file_capture=True, tail_log_lines=100
      ):
          with redirected_stdio(capture_stderr=True) as (stdout_path, stderr_path):
              ...
```

Differential Revision: D87488586

Author: shayne-fletcher