patch 8.0.1539: no test for the popup menu positioning

Problem:    No test for the popup menu positioning.
Solution:   Add a screendump test for the popup menu.
diff --git a/src/version.c b/src/version.c
index d434ea1..c75d290 100644
--- a/src/version.c
+++ b/src/version.c
@@ -779,6 +779,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1539,
+/**/
     1538,
 /**/
     1537,