Add more e2e tests for the different probes

[ReToCode]

Describe the feature

• We currently have very limited e2e testing of readiness & liveness probes
• We should add some more tests with various scenarios like
  • Readiness/liveness probes mixed
  • One container/multiple containers with probes
  • A mix of probes
  • Probes that are failing from the beginning
  • Probes that are successful first, then start failing
• The readiness server needs to be extended with endpoints to toggle readiness, e.g. like https://github.com/ReToCode/knative-multicontainer-probing/blob/main/test-server/main.go
• To toggle readiness of non-user containers we need to deploy a proxy to call other ports than the 8080 (UC port)

[mgencur]

/assign