Test suite: fix clicking the Print button in Tor Browser (refs: #15023)

Mixing Dogtail and Sikuli is tricky: in this case, Dogtail pretended it had
clicked the Print button, but that was before the output file selection dialog
had disappeared, so that click did nothing. So let's do the same as in the
Evince test, that works in a robust manner.

features/step_definitions/common_steps.rb

@@ -787,7 +787,7 @@ When /^I can print the current page as "([^"]+[.]pdf)" to the (default downloads
   $vm.set_clipboard(output_dir + '/' + output_file.sub(/[.]pdf$/, ''))
   @screen.type('v', Sikuli::KeyModifier.CTRL)
   @screen.type(Sikuli::Key.ENTER)
-  print_dialog.button('Print').click
+  @screen.wait_and_click("Gtk3PrintButton.png", 10)
   try_for(30, :msg => "The page was not printed to #{output_dir}/#{output_file}") {
     $vm.file_exist?("#{output_dir}/#{output_file}")
   }