patch 8.2.0789: Vim9: expression testing lost coverage using constants

Problem:    Vim9: expression testing lost coverage using constants.
Solution:   Use a few variables instead of constants.
diff --git a/src/version.c b/src/version.c
index 4dcae55..53d69d5 100644
--- a/src/version.c
+++ b/src/version.c
@@ -747,6 +747,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    789,
+/**/
     788,
 /**/
     787,