patch 8.2.4408: Vim9: some code not covered by tests

Problem:    Vim9: some code not covered by tests.
Solution:   Add a few more tests.  Correct error message.  Allow unlet on dict
            with a number key.

src/version.c

@@ -750,6 +750,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    4408,
 /**/
     4407,
 /**/