Add development-ish domains to redirect validation

- This allows common development urls to be considered valid on
experiment redirect urls. This improves development support.

config/initializers/gitlab_experiment.rb

@@ -42,7 +42,7 @@ Gitlab::Experiment.configure do |config|
   # This behavior doesn't make perfect sense for self managed installs either,
   # so we don't think we should redirect in those cases.
   #
-  valid_domains = %w[about.gitlab.com docs.gitlab.com gitlab.com]
+  valid_domains = %w[about.gitlab.com docs.gitlab.com gitlab.com gdk.test localhost]
   config.redirect_url_validator = lambda do |url|
     Gitlab.dev_env_or_com? && (url = URI.parse(url)) && valid_domains.include?(url.host)
   rescue URI::InvalidURIError

spec/experiments/application_experiment_spec.rb

@@ -251,6 +251,8 @@ RSpec.describe ApplicationExperiment, :experiment do
       "https://badplace.com\nhttps://gitlab.com"  | nil
       'https://gitlabbcom'                        | nil
       'https://gitlabbcom/'                       | nil
+      'http://gdk.test/foo/bar'                   | 'http://gdk.test/foo/bar'
+      'http://localhost:3000/foo/bar'             | 'http://localhost:3000/foo/bar'
     end
 
     with_them do