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)

src/screen.c

@@ -10580,7 +10580,12 @@ fillchar_vsep(int *attr)
     int
 redrawing(void)
 {
-    return (!RedrawingDisabled
+#ifdef FEAT_EVAL
+    if (disable_redraw_for_testing)
+	return 0;
+    else
+#endif
+	return (!RedrawingDisabled
 		       && !(p_lz && char_avail() && !KeyTyped && !do_redraw));
 }