Avoid simultaneous requests in moderation test

We were clicking on the "Sign in" link right after clicking on the "Sign
out" link, which might result in simultaneous requests and exceptions
when running our test suite.

So we're adding an expectation to make sure the first request has
finished before starting the following one.

spec/system/moderation/users_spec.rb

@@ -38,7 +38,9 @@ describe "Moderate users" do
 
     expect(page).not_to have_content(comment3.body)
 
-    click_link("Sign out")
+    click_link "Sign out"
+
+    expect(page).to have_content "You have been signed out successfully"
 
     visit root_path