patch 8.0.0440: not enough test coverage in Insert mode

Problem:    Not enough test coverage in Insert mode.
Solution:   Add lots of tests.  Add test_override(). (Christian Brabandt,
            closes #1521)
diff --git a/src/globals.h b/src/globals.h
index 831cbf1..f818663 100644
--- a/src/globals.h
+++ b/src/globals.h
@@ -1648,7 +1648,9 @@
 /* set by alloc_fail(), number of times alloc() returns NULL */
 EXTERN int  alloc_fail_repeat INIT(= 0);
 
+/* flags set by test_override() */
 EXTERN int  disable_char_avail_for_testing INIT(= 0);
+EXTERN int  disable_redraw_for_testing INIT(= 0);
 
 EXTERN int  in_free_unref_items INIT(= FALSE);
 #endif