message: test: fix bitcoind already running warnings on macOS
  On macOS, pidof installed via brew returns b'' rather than None.
  Account for this, to remove spurious warnings from the test_runner.
  Github-Pull:
  Rebased-From:
diff: @@ -361,9 +361,10 @@ def main():
  def run_tests(*, test_list, src_dir, build_dir, tmpdir, jobs=1, enable_coverage=False, args=None, combined_logs_len=0, failfast=False, runs_ci, use_term_control):
  args = args or []
  - # Warn if bitcoind is already running (unix only)
  + # Warn if bitcoind is already running
  + # pidof might fail or return an empty string if bitcoind is not running
  try:
  - if subprocess.check_output(["pidof", "bitcoind"]) is not None:
  + if subprocess.check_output(["pidof", "bitcoind"]) not in [b'']:
  print("%sWARNING!%s There is already a bitcoind process running on this system. Tests may fail unexpectedly due to resource contention!" % (BOLD[1], BOLD[0]))
  except (OSError, subprocess.SubprocessError):
  pass
  