Merge pull request #6097 from sjackman/test-pgrep

test: Avoid pgrep -q, not available with GNU pgrep

Library/Homebrew/test.rb

@@ -33,7 +33,7 @@ rescue Exception => e # rubocop:disable Lint/RescueException
   error_pipe.puts e.to_json
   error_pipe.close
   pid = Process.pid.to_s
-  if which("pgrep") && which("pkill") && system("pgrep", "-qP", pid)
+  if which("pgrep") && which("pkill") && system("pgrep", "-P", pid, out: :close)
     $stderr.puts "Killing child processes..."
     system "pkill", "-P", pid
     sleep 1