Add missing expectations before opening the last email

We weren't checking that the requests had finished before checking the
last sent email. That's probably why one of these tests has recently
failed on our CI:

```
1) System Emails Preview Pending #send_pending
  Failure/Error: email = open_last_email

  RuntimeError:
    No email has been sent!
```

spec/support/common_actions/users.rb

@@ -11,6 +11,7 @@ module Users
     check "user_terms_of_service"
 
     click_button "Register"
+    expect(page).to have_content "Thank you for registering"
   end
 
   def login_through_form_with_email_and_password(email = "manuela@consul.dev", password = "judgementday")

spec/system/admin/system_emails_spec.rb

@@ -359,6 +359,8 @@ describe "System Emails" do
 
       click_button "Send pending"
 
+      expect(page).to have_content "Pending notifications sent successfully"
+
       email = open_last_email
       expect(email).to deliver_to(voter)
       expect(email).to have_body_text(proposal_notification.body)