Work around screen blanking when testing Synaptic.

Updating APT may take more than the time needed for screen blanking
(and screen locking) to kick in. We prevent this by moving the mouse
every minute.

Refs: #10403

features/step_definitions/apt.rb

@@ -30,7 +30,15 @@ end
 When /^I update APT using Synaptic$/ do
   @screen.click('SynapticReloadButton.png')
   @screen.wait('SynapticReloadPrompt.png', 20)
-  @screen.waitVanish('SynapticReloadPrompt.png', 30*60)
+  try_for(30*60) do
+    begin
+      @screen.waitVanish('SynapticReloadPrompt.png', 60)
+    rescue Exception => e
+      @screen.hover_point(0, 0)
+      @screen.hide_cursor
+      raise e
+    end
+  end
   # After this next image is displayed, the GUI should be responsive.
   @screen.wait('SynapticPackageList.png', 30)
 end