patch 8.1.2421: test88 is old style

Problem:    Test88 is old style.
Solution:   Turn into a new style test. (Yegappan Lakshmanan, closes #5347)
diff --git a/src/version.c b/src/version.c
index 7b36fe3..340867d 100644
--- a/src/version.c
+++ b/src/version.c
@@ -743,6 +743,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    2421,
+/**/
     2420,
 /**/
     2419,