patch 8.2.4728: no test that v:event cannot be modified

Problem:    No test that v:event cannot be modified.
Solution:   Add a test. (closes #10139)

src/version.c

@@ -746,6 +746,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    4728,
 /**/
     4727,
 /**/