Fix test_balloon and disable GUI tests as they don't work in MacVim

Author: ychin

src/testdir/test_balloon.vim

@@ -58,6 +58,8 @@ endif
 " Tests that only work in the GUI
 if has('gui_running')
 
+if !has('gui_macvim') " See https://github.com/macvim-dev/macvim/issues/902
+
 func Test_balloon_show_gui()
   let msg = 'this this this this'
   call balloon_show(msg)
@@ -72,4 +74,6 @@ func Test_balloon_show_gui()
   call balloon_show('')
 endfunc
 
+endif " !has('gui_macvim')
+
 endif