[BUG][Test Framework] response.payload is set to null for non-2XX responses

[nhtruong]

Consider this 403 response:

    security.delete_distinguished_name@403:
      description: ''
      content:
        application/json:
          schema:
            type: object
            properties:
              status:
                type: string
                enum:
                  - 403
                  - FORBIDDEN
              message:
                type: string
                description: Message returned as part of Forbidden response.

When a 403 happens to a security.delete_distinguished_name request, the cluster also returns a response body with {"status":"FORBIDDEN","message":"Access denied"}. However testing this in a test story will result in this error when validating the response body:

[INFO] * undefined
[INFO] & "data must be object, data must be object"

That is the test framework thought the returned payload was null. This is a bug in the framework and most likely caused by this line:

if (payload !== undefined) response.payload = payload.error

The response.payload is set to payload.error while the payload itself is {"status":"FORBIDDEN","message":"Access denied"}, resulting in response.payload = undefined

Related to #439
@DarshitChanpura