patch 8.0.1320: popup test fails on GUI-only build

Problem:    Popup test fails on GUI-only build.
Solution:   Don't test balloon_split() when it's not available.

src/testdir/test_popup.vim

@@ -704,6 +704,9 @@ func Test_popup_and_preview_autocommand()
 endfunc
 
 func Test_balloon_split()
+  if !exists('*balloon_split')
+    return
+  endif
   call assert_equal([
         \ 'one two three four one two three four one two thre',
         \ 'e four',

src/version.c

@@ -771,6 +771,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1320,
 /**/
     1319,
 /**/