Fix flaky spec "Notifications User not logged in"

We had a test failing several times in GitHub Actions where a user was
still logged in even after logout.

This issue can be reproduced running:

```
rspec spec/system/moderation/proposal_notifications_spec.rb:71 \
spec/system/notifications_spec.rb:126 --order defined
```

One possible cause is a concurrency issue because the process running
the test and the process running the browser both access the same
database connection. Maybe some database operations leak between tests
due to that, particularly if the previous test accessed the database
after starting the browser as well.

A way to avoid this possible cause is setting up the database before
starting the browser with a call to `visit`.

spec/system/notifications_spec.rb

@@ -2,17 +2,14 @@ require "rails_helper"
 
 describe "Notifications" do
   let(:user) { create :user }
-
+  before { login_as(user) }
-  before do
-    login_as(user)
-    visit root_path
-  end
 
   scenario "View all" do
     read1 = create(:notification, :read, user: user)
     read2 = create(:notification, :read, user: user)
     unread = create(:notification, user: user)
 
+    visit root_path
     click_notifications_icon
     click_link "Read"
 
@@ -27,6 +24,7 @@ describe "Notifications" do
     unread2 = create(:notification, user: user)
     read = create(:notification, :read, user: user)
 
+    visit root_path
     click_notifications_icon
     click_link "Unread"
 
@@ -40,6 +38,7 @@ describe "Notifications" do
     proposal = create(:proposal)
     create(:notification, user: user, notifiable: proposal)
 
+    visit root_path
     click_notifications_icon
 
     first(".notification a").click
@@ -56,6 +55,7 @@ describe "Notifications" do
     notification1 = create(:notification, user: user)
     notification2 = create(:notification, user: user)
 
+    visit root_path
     click_notifications_icon
 
     within("#notification_#{notification1.id}") do
@@ -70,6 +70,7 @@ describe "Notifications" do
   scenario "Mark all as read" do
     2.times { create(:notification, user: user) }
 
+    visit root_path
     click_notifications_icon
 
     expect(page).to have_css(".notification", count: 2)
@@ -82,6 +83,7 @@ describe "Notifications" do
     notification1 = create(:notification, :read, user: user)
     notification2 = create(:notification, user: user)
 
+    visit root_path
     click_notifications_icon
     click_link "Read"
 
@@ -115,6 +117,7 @@ describe "Notifications" do
   end
 
   scenario "No notifications" do
+    visit root_path
     click_notifications_icon
     expect(page).to have_content "You don't have new notifications."
   end
@@ -129,6 +132,7 @@ describe "Notifications" do
   scenario "Notification's notifiable model no longer includes Notifiable module" do
     create(:notification, :for_poll_question, user: user)
 
+    visit root_path
     click_notifications_icon
     expect(page).to have_content("This resource is not available anymore.", count: 1)
   end