patch 8.1.1544: some balloon tests don't run when they can

Problem:    Some balloon tests don't run when they can.
Solution:   Split GUI balloon tests off into a separate file. (Ozaki Kiichi,
            closes #4538)  Change the feature check into a command for
            consistency.
diff --git a/src/testdir/test_menu.vim b/src/testdir/test_menu.vim
index 99f888a..efbb982 100644
--- a/src/testdir/test_menu.vim
+++ b/src/testdir/test_menu.vim
@@ -1,8 +1,7 @@
 " Test that the system menu can be loaded.
 
-if !has('menu')
-  throw 'Skipped, menu feature missing'
-endif
+source check.vim
+CheckFeature menu
 
 func Test_load_menu()
   try