Create only one poll for a poll voter

The factories were creating strange database relations:

* The voter belonged to a poll, to a booth and to an officer
* The booth belonged to a different poll
* The officer belonged to a different booth

The code uses an unusual syntax because the following code:

association :booth_assignment,
            factory: :poll_booth_assignment,
            poll: poll

Would generate the following error:

ActiveRecord::AssociationTypeMismatch: Poll(#46976420451940) expected,
got FactoryBot::Declaration::Implicit

spec/factories/polls.rb

@@ -92,7 +92,10 @@ FactoryBot.define do
 
     trait :from_booth do
       origin "booth"
-      association :booth_assignment, factory: :poll_booth_assignment
+
+      booth_assignment do
+        association :poll_booth_assignment, poll: poll
+      end
 
       officer_assignment do
         association :poll_officer_assignment, booth_assignment: booth_assignment, officer: officer