patch 8.2.4087: cannot test items from an autoload script easily

Problem:    Cannot test items from an autoload script easily.
Solution:   Add the "autoload" value for test_override().
diff --git a/src/globals.h b/src/globals.h
index f04b29e..94da8e0 100644
--- a/src/globals.h
+++ b/src/globals.h
@@ -1643,6 +1643,7 @@
 EXTERN int  ui_delay_for_testing INIT(= 0);
 EXTERN int  reset_term_props_on_termresponse INIT(= FALSE);
 EXTERN long override_sysinfo_uptime INIT(= -1);
+EXTERN int  override_autoload INIT(= FALSE);
 
 EXTERN int  in_free_unref_items INIT(= FALSE);
 #endif