Fix healthcheck test flakes (#28790)
Fix healthcheck test flakes (#28790)

* Use dynamic ports to avoid conflict
* Switch the sync mechanism to avoid race condition causing negative
waitgroup
(https://prow.istio.io/view/gs/istio-prow/pr-logs/pull/istio_istio/28598/unit-tests_istio/1326201514657583104)
istio/istio,istio/istio,istio/istio,istio/istio,istio/istio