patch 8.1.1426: no test for syntax highlight in popup window

Problem:    No test for syntax highlight in popup window.
Solution:   Add a screenshot test.  Update associated documentation. Avoid
            'buftype' being reset by setbufvar().

src/version.c

@@ -767,6 +767,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1426,
 /**/
     1425,
 /**/