Complete proposal notification test

We were finishing the test with the first "visit", so it was doing
nothing (other than potentially generating concurrency issues with other
tests).

spec/system/proposal_notifications_spec.rb

@@ -313,12 +313,24 @@
  scenario "Proposal retired by author" do
  author = create(:user)
  user = create(:user)
- proposal = create(:proposal, author: author, voters: [user])
+ proposal = create(:proposal, :retired, author: author, followers: [user])
 
  login_as(author)
- visit root_path
 
  visit new_proposal_notification_path(proposal_id: proposal.id)
+
+ fill_in "proposal_notification_title", with: "Thank you for supporting my proposal"
+ fill_in "proposal_notification_body", with: "Please share it with "\
+ "others so we can make it happen!"
+ click_button "Send notification"
+
+ expect(page).to have_content "Your message has been sent correctly."
+
+ logout
+ login_as user
+ visit notifications_path
+
+ expect(page).to have_content "This resource is not available anymore"
  end
 
  context "Group notifications" do