patch 8.2.3421: a bit of code is not covered by tests

Problem:    A bit of code is not covered by tests.
Solution:   Add a few more test cases. (Dominique Pellé, closes #8857)
diff --git a/src/version.c b/src/version.c
index 6d64ffe..d1767f3 100644
--- a/src/version.c
+++ b/src/version.c
@@ -756,6 +756,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    3421,
+/**/
     3420,
 /**/
     3419,