Preserve installed MAME settings in isolated ordinary runs

tests/sdbg/test_launcher.py

@@ -60,5 +60,25 @@ class LauncherTests(unittest.TestCase):
             self.assertEqual(keyboards, {':': '1', ':kbd:ms_naturl': '1'})
 
 
+    def test_existing_config_is_merged_not_replaced(self):
+        with tempfile.TemporaryDirectory() as directory:
+            path = Path(directory)/'sprinter.cfg'
+            path.write_text('\ufeff<?xml version="1.0"?>\n<mameconfig version="10">'
+                            '<system name="sprinter"><input>'
+                            '<keyboard tag=":" enabled="0" />'
+                            '<port tag=":TURBO" type="OTHER" mask="1" defvalue="0" value="1" />'
+                            '</input><video><target index="0" /></video>'
+                            '</system></mameconfig>', encoding='utf-8')
+            write_keyboard_config(Path(directory))
+            root = ET.parse(path).getroot()
+            system = root.find('system')
+            self.assertEqual([item.tag for item in system.find('input')],
+                             ['keyboard', 'keyboard', 'port'])
+            keyboards = {item.attrib['tag']: item.attrib['enabled']
+                         for item in system.findall('input/keyboard')}
+            self.assertEqual(keyboards, {':': '1', ':kbd:ms_naturl': '1'})
+            self.assertEqual(system.find('input/port').attrib['value'], '1')
+            self.assertIsNotNone(system.find('video/target'))
+
 if __name__ == '__main__':
     unittest.main()