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
self.child.cmdline.return_value = [settings.GUNICORN_WORKER_READY_PREFIX]
        self.process.children.return_value = [self.child]
        with mock.patch('psutil.Process', return_value=self.process):
            self.assertEqual(self.monitor._get_num_ready_workers_running(), 1)
    def test_ready_prefix_on_cmdline_no_children(self):
        self.process.children.return_value = []
        with mock.patch('psutil.Process', return_value=self.process):
            self.assertEqual(self.monitor._get_num_ready_workers_running(), 0)
    def test_ready_prefix_on_cmdline_zombie(self):
        self.child.cmdline.return_value = []
        self.process.children.return_value = [self.child]
        with mock.patch('psutil.Process', return_value=self.process):
            self.assertEqual(self.monitor._get_num_ready_workers_running(), 0)
    def test_ready_prefix_on_cmdline_dead_process(self):
        self.child.cmdline.side_effect = psutil.NoSuchProcess(11347)
        self.process.children.return_value = [self.child]
        with mock.patch('psutil.Process', return_value=self.process):
            self.assertEqual(self.monitor._get_num_ready_workers_running(), 0)
class TestCliWebServer(unittest.TestCase):
    @classmethod
    def setUpClass(cls):