Prevent factories in background migration specs

The Background migration specs also run against a certain version of
the database. So we should avoid using factories.

rubocop/spec_helpers.rb

 module RuboCop
   module SpecHelpers
     SPEC_HELPERS = %w[fast_spec_helper.rb rails_helper.rb spec_helper.rb].freeze
+    MIGRATION_SPEC_DIRECTORIES = ['spec/migrations', 'spec/lib/gitlab/background_migration'].freeze
 
     # Returns true if the given node originated from the spec directory.
     def in_spec?(node)
@@ -10,14 +11,18 @@ module RuboCop
         path.start_with?(File.join(Dir.pwd, 'spec'), File.join(Dir.pwd, 'ee', 'spec'))
     end
 
+    def migration_directories
+      @migration_directories ||= MIGRATION_SPEC_DIRECTORIES.map do |dir|
+        [File.join(Dir.pwd, dir), File.join(Dir.pwd, 'ee', dir)]
+      end.flatten
+    end
+
     # Returns true if the given node originated from a migration spec.
     def in_migration_spec?(node)
       path = node.location.expression.source_buffer.name
 
       in_spec?(node) &&
-        path.start_with?(
-          File.join(Dir.pwd, 'spec', 'migrations'),
-          File.join(Dir.pwd, 'ee', 'spec', 'migrations'))
+        path.start_with?(*migration_directories)
     end
   end
 end