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
with tempfile.TemporaryDirectory(prefix='tmp-pid') as tmpdir:
            pidfile = "{}/pidfile".format(tmpdir)
            with mock.patch.dict(
                "os.environ",
                AIRFLOW__CORE__DAGS_FOLDER="/dev/null",
                AIRFLOW__CORE__LOAD_EXAMPLES="False",
                AIRFLOW__WEBSERVER__WORKERS="1"
            ):
                proc = subprocess.Popen(["airflow", "webserver", "--pid", pidfile])
                self.assertEqual(None, proc.poll())
            
            self._wait_pidfile(pidfile)
            
            proc.terminate()
            self.assertEqual(0, proc.wait(60))
    @pytest.mark.quarantined
    def test_cli_webserver_background(self):