patch 8.2.3839: using \z() with \z1 not tested for syntax highlighting

Problem:    Using \z() with \z1 not tested for syntax highlighting.
Solution:   Add a test. (Dominique Pellé, closes #9365)

src/version.c

@@ -749,6 +749,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    3839,
 /**/
     3838,
 /**/