We have some random failures in teardown of TestCliWebServer.test_cli_webserver_foreground.

Since this is in teardown, it can be likely ignored because we are killing those background processes with -9 anyway.

https://github.com/apache/airflow/pull/11526/checks?check_run_id=1268530821#step:6:1193

```
________________ TestCliWebServer.test_cli_webserver_foreground ________________

self = <tests.cli.commands.test_webserver_command.TestCliWebServer testMethod=test_cli_webserver_foreground>

    def tearDown(self) -> None:
>       self._check_processes()

tests/cli/commands/test_webserver_command.py:247: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

self = <tests.cli.commands.test_webserver_command.TestCliWebServer testMethod=test_cli_webserver_foreground>

    def _check_processes(self):
        # Confirm that webserver hasn't been launched.
        # pgrep returns exit status 1 if no process matched.
        exit_code_pgrep_webserver = subprocess.Popen(["pgrep", "-c", "-f", "airflow webserver"]).wait()
        exit_code_pgrep_gunicorn = subprocess.Popen(["pgrep", "-c", "-f", "gunicorn"]).wait()
        if exit_code_pgrep_webserver != 1 or exit_code_pgrep_gunicorn != 1:
            subprocess.Popen(["ps", "-ax"]).wait()
            if exit_code_pgrep_webserver != 1:
                subprocess.Popen(["pkill", "-9", "-f", "airflow webserver"]).wait()
            if exit_code_pgrep_gunicorn != 1:
                subprocess.Popen(["pkill", "-9", "-f", "gunicorn"]).wait()
    
            raise AssertionError(
>               "Background processes are running that prevent the test from passing successfully."
            )
E           AssertionError: Background processes are running that prevent the test from passing successfully.

tests/cli/commands/test_webserver_command.py:243: AssertionError
```
tests/cli/commands/test_webserver_command.py
"webserver",
                    "--daemon",
                    "--pid", pidfile_webserver,
                    "--stdout", stdout,
                    "--stderr", stderr,
                    "--log-file", logfile,
                ])
                self.assertEqual(None, proc.poll())
                pid_monitor = self._wait_pidfile(pidfile_monitor)
                self._wait_pidfile(pidfile_webserver)
                
                self.assertEqual(
                    0, subprocess.Popen(["pgrep", "-f", "-c", "airflow webserver --daemon"]).wait()
                )
                self.assertEqual(0, subprocess.Popen(["pgrep", "-c", "-f", "gunicorn: master"]).wait())
                
                proc = psutil.Process(pid_monitor)
                proc.terminate()
                self.assertIn(proc.wait(120), (0, None))
                self._check_processes()
            except Exception:
                
                subprocess.Popen(["ls", "-lah", tmpdir]).wait()
                
                subprocess.Popen(["bash", "-c", "ls {}/* | xargs -n 1 -t cat".format(tmpdir)]).wait()
                raise
    
    @mock.patch("airflow.cli.commands.webserver_command.GunicornMonitor._get_num_workers_running",
                return_value=0)
    def test_cli_webserver_shutdown_when_gunicorn_master_is_killed(self, _):