Upgrade tests to be added into this repo to test the upgrade of operator with knative serving #287

Describe the bug
This repo does not have test suit dedicated to upgrade.

Expected behavior
The ultimate goal is to run the upgrade tests in serving, tagged now with preupgrade, postupgrade, etc here in this operator repo. The current prow job named pull-knative-serving-operator-integration-tests-on-latest-serving can be converted to upgrade tests.

To Reproduce
Steps to reproduce the behavior.

Knative release version
The Knative version, where you find this issue.

Additional context
Add any other context about the problem here such as proposed priority