patch 8.2.0982: insufficient testing for reading/writing files

Problem:    Insufficient testing for reading/writing files.
Solution:   Add more tests. (Yegappan Lakshmanan, closes #6257)
            Add "ui_delay" to test_override() and use it for the CTRL-O test.
diff --git a/src/globals.h b/src/globals.h
index f0b75e5..cb85bcc 100644
--- a/src/globals.h
+++ b/src/globals.h
@@ -1841,6 +1841,7 @@
 EXTERN int  ignore_redraw_flag_for_testing INIT(= FALSE);
 EXTERN int  nfa_fail_for_testing INIT(= FALSE);
 EXTERN int  no_query_mouse_for_testing INIT(= FALSE);
+EXTERN int  ui_delay_for_testing INIT(= 0);
 EXTERN int  reset_term_props_on_termresponse INIT(= FALSE);
 
 EXTERN int  in_free_unref_items INIT(= FALSE);