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.
diff --git a/src/testdir/test_popup.vim b/src/testdir/test_popup.vim
index bfccb74..2bb9b32 100644
--- a/src/testdir/test_popup.vim
+++ b/src/testdir/test_popup.vim
@@ -704,6 +704,9 @@
 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',