By using errgroup.WithContext, all probes are cancelled on the first
error returned.  This changes to use an errgroup without a
context/cancellation.  So all probes are allowed to run to completion
and one failed probe does not cause all probes to exit.

Ref knative#14200

- Add pod IP probe tests to directly test the pod IP probing behavior
  - Add test for no-probe optimization when all healthy
  - Add test for a pod returning an error to verify it does not block /
    cancel other probes (confirmed test with prior code which fails)
- Update the fake roundtripper to support the new pod IP probe tests by
  introducing the Delay field to be used to delay the response.  Default
  handling skips the delay if not set.
- Add comment to errgroup change since this had been correct before and
  was incorrectly changed. (Additionally change to use original form.)

Update to use a function for the delay to reduce duplicate code.