Test suite: make image reusable.

It's actually the same dialog for all GTK3 apps and e.g. Tor Browser 8
can now reuse it.

features/step_definitions/evince.rb

@@ -7,7 +7,7 @@ Then /^I can print the current document to "([^"]+)"$/ do |output_file|
   @screen.wait("EvincePrintDialog.png", 10)
   @screen.wait_and_click("EvincePrintToFile.png", 10)
   @screen.wait_and_click("EvincePrintOutputFileButton.png", 10)
-  @screen.wait("EvincePrintFileDialog.png", 10)
+  @screen.wait("Gtk3PrintFileDialog.png", 10)
   # Only the file's basename is selected by double-clicking,
   # so we type only the desired file's basename to replace it
   $vm.set_clipboard(output_file.sub(/[.]pdf$/, ''))