Use logout to sign out in tests

We were doing it most of the time, but in some cases we were clicking
the "Sign out" link instead. These actions aren't the same, just like
using `login_as` is not the same as visiting the sign in page and
submitting the form.

Some of these tests failed sometimes because the user wasn't signed in
after using `login_as`. One possible cause could be that we weren't
adding an expectation after clicking the "Sign out" link.

So using `logout` adds consistency, simplifies the code, and might
reduce the chance of these tests failing in the future (although they
might still fail in the future because some of these tests check the
database after a `visit` call).

spec/system/budget_polls/voter_spec.rb

@@ -32,8 +32,7 @@ describe "BudgetPolls", :with_frozen_time do
       expect(Poll::Voter.count).to eq(1)
       expect(Poll::Voter.first.origin).to eq("booth")
 
-      visit root_path
+      logout
-      click_link "Sign out"
       login_as(admin.user)
       visit admin_poll_recounts_path(poll)
 

spec/system/officing/voters_spec.rb

@@ -74,8 +74,7 @@ describe "Voters" do
                   document_number: "12345678Z")
     expect(user).not_to be_level_two_verified
 
-    visit root_path
+    logout
-    click_link "Sign out"
     login_through_form_as_officer(officer.user)
 
     visit new_officing_residence_path

spec/system/polls/voter_spec.rb

@@ -90,8 +90,7 @@ describe "Voter" do
       expect(Poll::Voter.count).to eq(1)
       expect(Poll::Voter.first.origin).to eq("booth")
 
-      visit root_path
+      logout
-      click_link "Sign out"
       login_as(admin.user)
       visit admin_poll_recounts_path(poll)
 
@@ -148,8 +147,7 @@ describe "Voter" do
         vote_for_poll_via_web(poll, question, "Yes")
         expect(Poll::Voter.count).to eq(1)
 
-        click_link "Sign out"
+        logout
-
         login_through_form_as_officer(officer.user)
 
         visit new_officing_residence_path
@@ -165,9 +163,7 @@ describe "Voter" do
 
         vote_for_poll_via_booth
 
-        visit root_path
+        logout
-        click_link "Sign out"
-
         login_as user
         visit poll_path(poll)
 
@@ -178,8 +174,7 @@ describe "Voter" do
                                      "You can not participate again."
         expect(Poll::Voter.count).to eq(1)
 
-        visit root_path
+        logout
-        click_link "Sign out"
         login_as(admin.user)
         visit admin_poll_recounts_path(poll)
 
@@ -199,9 +194,7 @@ describe "Voter" do
       login_through_form_as_officer(officer.user)
       vote_for_poll_via_booth
 
-      visit root_path
+      logout
-      click_link "Sign out"
-
       login_as user
       visit account_path
       click_link "Verify my account"
@@ -219,8 +212,7 @@ describe "Voter" do
                                    "You can not participate again."
       expect(Poll::Voter.count).to eq(1)
 
-      visit root_path
+      logout
-      click_link "Sign out"
       login_as(admin.user)
       visit admin_poll_recounts_path(poll)