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

spec/serializers/rest/poll_serializer_spec.rb

@@ -15,11 +15,14 @@ RSpec.describe REST::PollSerializer do
   end
 
   let(:current_user) { Fabricate(:user) }
-  let(:poll) { Fabricate :poll }
+  let(:poll) { Fabricate.build :poll, expires_at: 5.days.from_now }
 
   context 'when expires_at is populated' do
     it 'parses as RFC 3339 datetime' do
-      expect { DateTime.rfc3339(subject['expires_at']) }.to_not raise_error
+      expect(subject)
+        .to include(
+          'expires_at' => match_api_datetime_format
+        )
     end
   end
 end