patch 8.2.4248: no proper test for moving the window separator

Problem:    No proper test for moving the window separator.
Solution:   Add a test.  Add comment in code. (closes #9656)
diff --git a/src/version.c b/src/version.c
index 005a394..13de4a4 100644
--- a/src/version.c
+++ b/src/version.c
@@ -751,6 +751,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    4248,
+/**/
     4247,
 /**/
     4246,