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.

src/testdir/test_lua.vim

@@ -1,8 +1,7 @@
 " Tests for Lua.
 
-if !has('lua')
-  throw 'Skipped, lua feature missing'
-endif
+source check.vim
+CheckFeature lua
 
 func TearDown()
   " Run garbage collection after each test to exercise luaV_setref().