Run upgrade tests in separate Prow check

[cardil]

Problem
Now, that PR #2388 is merged the upgrade tests are executed in the same run as regular e2e tests, sequential. It isn't perfect as the log is double the size and it's harder to follow.

Example of such run can be seen here: https://prow.knative.dev/view/gcs/knative-prow/pr-logs/pull/knative_eventing/2388/pull-knative-eventing-integration-tests/1263045362487136256

Persona:
Which persona is this feature for? Contributors

Exit Criteria
There are separate Prow runs defined, one for regular e2e tests, and one for upgrade tests. Regular e2e test run should not include the upgrade tests.

Time Estimate (optional):
1d

Additional context (optional)
It was discussed before #2388 (comment)

[grantr]

@chaodaiG should we open an issue in test-infra to track this?

[chaodaiG]

Doesn't seem like a bug or feature request in test-infra to me. Please follow the way how serving separates tests in presubmit here: https://github.com/knative/test-infra/blob/025029c28e21216f8eaa7edb6ec9dfd34b031c2c/config/prod/prow/config_knative.yaml#L38

[cardil]

/assign @cardil