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.
diff --git a/src/version.c b/src/version.c
index 76d7895..edac690 100644
--- a/src/version.c
+++ b/src/version.c
@@ -751,6 +751,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    4408,
+/**/
     4407,
 /**/
     4406,