Getting a 403 error in REST
Have you got correct permissions for anonymous users to access the resource ?

testing.dev/drupal8/admin/people/permissions 
under the RESTFul Web Services ensure that Access GET on Content resource is ticked for Anonymous User. Clear cache and try your request again. 

Also you might not have the format type exposed for that resource in your rest.settings.yml file. This file sets the supported_formats and auth. Here is what is should look like in your case:

resources:
  'entity:node':
    GET:
      supported_formats:
        - hal_json
        - json
      supported_auth:
        - cookie