tests: add test case for missing plymouth binary

If there's no /usr/bin/plymouth then subprocess.call will raise OSError;
we should have a test case that makes sure that we handle this cleanly.

We actually fail this test right now, which illustrates issue #18.

tests/test_system_upgrade.py

@@ -33,6 +33,11 @@ def test_ping_when_dead(self, call):
         self.assertEqual(call.call_count, 2)
         self.assertTrue(self.ply.alive)
 
+    def test_mode_no_plymouth(self, call):
+        call.side_effect = OSError(2, 'No such file or directory')
+        self.ply.set_mode("updates")
+        self.assertFalse(self.ply.alive)
+
     def test_message(self, call):
         self.ply.message(self.msg)
         call.assert_called_once_with(self.msg_args)