fix small timing issue in test

lib/capybara/spec/session/window/window_opened_by_spec.rb

@@ -72,6 +72,7 @@ Capybara::SpecHelper.spec '#window_opened_by', requires: [:windows] do
     expect do
       @session.window_opened_by do
         button.click
+        sleep 0.1 # It's possible for window_opened_by to be fullfilled before the second window opens
       end
     end.to raise_error(Capybara::WindowError, two_windows_message)
     @session.document.synchronize(2, errors: [Capybara::CapybaraError]) do