Add API datetime/format matcher for serialization specs, reduce factories (#33325)

spec/serializers/rest/account_serializer_spec.rb

@@ -52,7 +52,10 @@ RSpec.describe REST::AccountSerializer do
     end
 
     it 'parses as RFC 3339 datetime' do
-      expect { DateTime.rfc3339(subject['created_at']) }.to_not raise_error
+      expect(subject)
+        .to include(
+          'created_at' => match_api_datetime_format
+        )
     end
   end