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_ruby.vim b/src/testdir/test_ruby.vim
index dcfdc62..f679bbd 100644
--- a/src/testdir/test_ruby.vim
+++ b/src/testdir/test_ruby.vim
@@ -1,8 +1,7 @@
 " Tests for ruby interface
 
-if !has('ruby')
-  throw 'Skipped, ruby feature missing'
-end
+source check.vim
+CheckFeature ruby
 
 func Test_ruby_change_buffer()
   call setline(line('$'), ['1 line 1'])