flaky test: httpbin authn

[craigbox]

https://istio.io/latest/docs/tasks/security/authentication/authn-policy/

httpbin is returning errors when it should be returning a 200. This might be because it is not up by the time the test is run?

https://prow.istio.io/view/gs/istio-prow/pr-logs/pull/istio_istio.io/16515/doc.test.profile-default_istio.io/1927576459224813568

https://storage.googleapis.com/istio-prow/pr-logs/pull/istio_istio.io/16515/doc.test.profile-default_istio.io/1927576459224813568/artifacts/tests-setup-profile-default-d60/TestDocs/tasks/security/authentication/authn-policy/gtwapi_test.sh/gtwapi_test.sh/_test_context/gtwapi_test.sh_output.txt (has a 503 instead of a 200)

https://storage.googleapis.com/istio-prow/pr-logs/pull/istio_istio.io/16515/doc.test.profile-default_istio.io/1927576459224813568/artifacts/tests-setup-profile-default-d60/TestDocs/tasks/security/authentication/authn-policy/test.sh/test.sh/_test_context/test.sh_output.txt (has a 000 instead of a 200)