Someties test_cli_webserver_background sometimes detect processes left behind

### Body Sometimes (rather rarely) the "check_processes" in `tests/cli/commands/test_webserver_command.py::TestCliWebServer::test_cli_webserver_background` detects some remaining processes. This is likely a side effect of other test. Example failure: https://github.com/ache/airflow/actions/runs/6996129264/job/19031743272?pr=35862 ``` ________________ TestCliWebServer.test_cli_webserver_background ________________ self = <tests.cli.commands.test_webserver_command.TestCliWebServer object at 0x7f2352c76e20> tmp_path = PosixPath('/tmp/pytest-of-root/pytest-0/test_cli_webserver_background0') @pytest.mark.execution_timeout(210) def test_cli_webserver_background(self, tmp_path): with mock.patch.dict( "os.environ", AIRFLOW__CORE__DAGS_FOLDER="/dev/null", AIRFLOW__CORE__LOAD_EXAMPLES="False", AIRFLOW__WEBSERVER__WORKERS="1", ): pidfile_webserver = tmp_path / "pidflow-webserver.pid" pidfile_monitor = tmp_path / "pidflow-webserver-monitor.pid" stdout = tmp_path / "airflow-webserver.out" stderr = tmp_path / "airflow-webserver.err" logfile = tmp_path / "airflow-webserver.log" try: # Run webserver as daemon in background. Note that the wait method is not called. proc = subprocess.Popen( [ "airflow", "webserver", "--daemon", "--pid", os.fspath(pidfile_webserver), "--stdout", os.fspath(stdout), "--stderr", os.fspath(stderr), "--log-file", os.fspath(logfile), ] ) assert proc.poll() is None pid_monitor = self._wait_pidfile(pidfile_monitor) …