patch 8.1.0986: rename() is not propertly tested

Problem:    rename() is not propertly tested.
Solution:   Add tests. (Dominique Pelle, closes #4061)
diff --git a/src/testdir/Make_all.mak b/src/testdir/Make_all.mak
index 7cc5e7e..774c96b 100644
--- a/src/testdir/Make_all.mak
+++ b/src/testdir/Make_all.mak
@@ -212,6 +212,7 @@
 	test_regexp_utf8 \
 	test_registers \
 	test_reltime \
+	test_rename \
 	test_restricted \
 	test_retab \
 	test_ruby \