tests: mark test_deleted_cert as xfail

test_delete_cert_and_restart_reverseproxy still fails intermitently

test/stress_tests/test_deleted_cert/test_restart_while_missing_cert.py

@@ -9,6 +9,7 @@ from requests import ConnectionError
 
 script_dir = os.path.dirname(__file__)
 
+pytestmark = pytest.mark.xfail()  # TODO delete this marker once those issues are fixed
 
 @pytest.fixture(scope="module", autouse=True)
 def certs():