Document the expected validation of factories and its traits

doc/development/testing_guide/best_practices.md

@@ -986,6 +986,7 @@ GitLab uses [factory_bot](https://github.com/thoughtbot/factory_bot) as a test f
   See [issue #262624](https://gitlab.com/gitlab-org/gitlab/-/issues/262624) for further context.
 - Factories don't have to be limited to `ActiveRecord` objects.
   [See example](https://gitlab.com/gitlab-org/gitlab-foss/commit/0b8cefd3b2385a21cfed779bd659978c0402766d).
+- Factories and their traits should produce valid objects that are [verified by specs](https://gitlab.com/gitlab-org/gitlab/-/blob/master/spec/factories_spec.rb).
 
 ### Fixtures
 

spec/factories_spec.rb

@@ -5,6 +5,10 @@ require 'spec_helper'
 RSpec.describe 'factories' do
   include Database::DatabaseHelpers
 
+  # https://gitlab.com/groups/gitlab-org/-/epics/5464 tracks the remaining
+  # skipped traits.
+  #
+  # Consider adding a code comment if a trait cannot produce a valid object.
   def skipped_traits
     [
       [:audit_event, :unauthenticated],