Fixes #14435

Proposed Changes

• Wait for the HTTPS endpoint to be available to avoid flaky HTTPS E2E tests
  • KSvc is marked as Ready when the HTTP endpoint is available, even if the HTTPS endpoint is not Ready yet due to cert-manager being slow/under load
  • E2E tests start producing load on the HTTP endpoint
  • when the Certificate is Ready, KSvc becomes Ready and the HTTPS endpoint is set in the status
  • Switch to HTTPS is not as graceful as it seems when Istio creates a Gateway for access from external via HTTPS

Testing

I tried to reproduce it locally but didn't see the exact error message. I was able get similar ones though by replicating the E2E environment, scale cert-manager to 0, start the test and scale cert-manager up again after ~10sec or when the logs show errors (happens only in the unfixed code).
Requirement to test is a config-network like this and an appropriate cert-manager setup as in the E2E tests which is quite complicated, let me know if you need it.

apiVersion: v1
kind: ConfigMap
metadata:
  name: config-network
  namespace: knative-serving
data:
  autocreate-cluster-domain-claims: "true"
  cluster-local-domain-tls: Disabled
  external-domain-tls: Enabled
  system-internal-tls: Disabled

I also thought about adding some kind of polling to start the tests only when the first HTTPS request was successful, I'll add it in case this PR is not enough.

Release Note

Fix flaky HTTPS e2e tests by waiting for HTTPS endpoint to be Ready.