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
def setUp(self) -> None:
        self.monitor = GunicornMonitor(
            gunicorn_master_pid=1,
            num_workers_expected=4,
            master_timeout=60,
            worker_refresh_interval=60,
            worker_refresh_batch_size=2,
            reload_on_plugin_change=True,
        )
        mock.patch.object(self.monitor, '_generate_plugin_state', return_value={}).start()
        mock.patch.object(self.monitor, '_get_num_ready_workers_running', return_value=4).start()
        mock.patch.object(self.monitor, '_get_num_workers_running', return_value=4).start()
        mock.patch.object(self.monitor, '_spawn_new_workers', return_value=None).start()
        mock.patch.object(self.monitor, '_kill_old_workers', return_value=None).start()
        mock.patch.object(self.monitor, '_reload_gunicorn', return_value=None).start()
    @mock.patch('airflow.cli.commands.webserver_command.sleep')
    def test_should_wait_for_workers_to_start(self, mock_sleep):