patch 8.2.1686: Vim9: "const!" not sufficiently tested

Problem:    Vim9: "const!" not sufficiently tested.
Solution:   Add a few more test cases.  Fix type checking.
diff --git a/src/version.c b/src/version.c
index f958df9..a0a6983 100644
--- a/src/version.c
+++ b/src/version.c
@@ -751,6 +751,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1686,
+/**/
     1685,
 /**/
     1684,