Gateway API test flakes

[craigbox]

https://prow.istio.io/view/gs/istio-prow/pr-logs/pull/istio_istio.io/15483/doc.test.profile-default_istio.io/1825705272044163072

The first failure is in TestDocs/tasks/traffic-management/ingress/ingress-control/gtwapi_test.sh.

Cleanup runs, with output looking similar to non-flaky runs. Then, the failure is that the snapshot differs by the continued presence of httpbin-gateway, which should have been removed in the cleanup.

There's a smelly TODO and sleep 10 in the cleanup.

A half a dozen more tests run and pass, then TestDocs/tasks/traffic-management/mirroring/gtwapi_test.sh runs. It removes its own httpbin HTTPRoute, thus failing its own before/after for the reverse reason.

(Link to prior discussion of similar flakes.)