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/Makefile b/src/Makefile
index 2df360a..0238836 100644
--- a/src/Makefile
+++ b/src/Makefile
@@ -2111,6 +2111,7 @@
 	test_digraph \
 	test_functions \
 	test_display \
+	test_edit \
 	test_ex_undo \
 	test_execute_func \
 	test_expand \