Simplify assigning booth to a voter

Unless we're using the booth assignment for something else in the test,
when creating a voter from booth, specifying the poll and the booth is
enough to create a booth assignment.

spec/factories/polls.rb

@@ -202,8 +202,10 @@ FactoryBot.define do
     trait :from_booth do
       origin { "booth" }
 
+      transient { booth { association(:poll_booth) } }
+
       booth_assignment do
-        association :poll_booth_assignment, poll: poll
+        association :poll_booth_assignment, poll: poll, booth: booth
       end
 
       officer_assignment do

spec/models/poll/voter_spec.rb

@@ -51,28 +51,24 @@ describe Poll::Voter do
     end
 
     it "is not valid if the user has already voted in different booth in the same poll" do
-      booth_assignment1 = create(:poll_booth_assignment, poll: poll)
-      booth_assignment2 = create(:poll_booth_assignment, poll: poll)
-
       user = create(:user, :level_two)
 
-      voter1 = create(:poll_voter, user: user, poll: poll, booth_assignment: booth_assignment1)
-      voter2 = build(:poll_voter, user: user, poll: poll, booth_assignment: booth_assignment2)
+      create(:poll_voter, :from_booth, user: user, poll: poll, booth: create(:poll_booth))
 
-      expect(voter2).not_to be_valid
-      expect(voter2.errors.messages[:document_number]).to eq(["User has already voted"])
+      voter = build(:poll_voter, :from_booth, user: user, poll: poll, booth: booth)
+
+      expect(voter).not_to be_valid
+      expect(voter.errors.messages[:document_number]).to eq(["User has already voted"])
     end
 
     it "is valid if the user has already voted in the same booth in different poll" do
-      booth_assignment1 = create(:poll_booth_assignment, booth: booth)
-      booth_assignment2 = create(:poll_booth_assignment, booth: booth, poll: poll)
-
       user = create(:user, :level_two)
 
-      voter1 = create(:poll_voter, user: user, booth_assignment: booth_assignment1)
-      voter2 = build(:poll_voter, user: user, booth_assignment: booth_assignment2)
+      create(:poll_voter, :from_booth, user: user, booth: booth, poll: create(:poll))
 
-      expect(voter2).to be_valid
+      voter = build(:poll_voter, :from_booth, user: user, booth: booth, poll: poll)
+
+      expect(voter).to be_valid
     end
 
     it "is not valid if the user has voted via web" do