Improve test_logging

Fix several issues in test_logging.py :

- Update the PYTHONPATH in order to make sure to use the tested
  version of Circus in the child process (was an issue when
  testing a development version of Circus, not installed in the
  system)
- Use Popen.terminate instead of using circusctl
- Use sys.executable instead of "python"
- Use a python script instead of bash
- Don't respawn the process instead of using a while loop
- Remove debugging comments