python/tests/test_actors.py: use context mgr for config (#1946)

Summary:
Pull Request resolved: #1946

following up on mariusae 's comment (https://www.internalfb.com/diff/D87449483?dst_version_fbid=2613910452329247&transaction_fbid=2002832840481448)

remove manual config save/restore boilerplate:
```
  config = get_configuration()
  enable_log_forwarding = config["enable_log_forwarding"]
  # ... etc
  configure(enable_log_forwarding=True, ...)
  try:
      # test code
  finally:
      configure(enable_log_forwarding=enable_log_forwarding, ...)
```
replace with a context manager phrasing:
```
  with configured(enable_log_forwarding=True, enable_file_capture=True, tail_log_lines=100):
      # test code
```
i intend to follow this diff up with one for the FD + sys.stdout/sys.stderr redirection

Reviewed By: thedavekwon

Differential Revision: D87478665

fbshipit-source-id: 80e65ee72be72ee1e7ee2aa352600566fc6f0e4c

Author: shayne-fletcher