Inconsistency with wait.PollImmediate and WaitForEndpointState

/area test-and-release
/kind bug
## Expected Behavior
`wait.PollImmediate` should keep on polling if I pass response checkers through `WaitForEndpointState` https://github.com/knative/pkg/blob/main/test/request.go#L78 until the state is reached or timeout happens.
## Actual Behavior
Since the response checkers for example: https://github.com/knative/pkg/blob/main/test/spoof/response_checks.go#L33 returns true or error, it makes `wait.PollImmediate` stop polling, that is - only the first endpoint state is checked.

## Additional Info
Apologies if I am reading the code wrong, but if this really is an inconsistency, I think the return should be changed to `false, nil` (in the statuscode check function as well), this makes sense according to me because anyway `WaitForEndpointState` is the only function which is propogating the response checkers to the Poll method, for `CheckEndpointState` its defaulted to `true, nil` and it makes sense because this method's scope is to just check once. If this reasoning makes sense, I can create a PR.

Start with test/request.go at WaitForEndpointState and compare its use of wait.PollImmediate with CheckEndpointState. Read test/spoof/response_checks.go, including the response checker behavior, and trace the status-code check. Done means endpoint states continue being checked until the target state or timeout, with relevant tests confirming the behavior.

Written by the indexing model from the issue text.