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_listlbr.vim b/src/testdir/test_listlbr.vim
index 5af4133..a2abc93 100644
--- a/src/testdir/test_listlbr.vim
+++ b/src/testdir/test_listlbr.vim
@@ -3,12 +3,9 @@
 set encoding=latin1
 scriptencoding latin1
 
-if !exists("+linebreak")
-  throw 'Skipped, linebreak option missing'
-endif
-if !has("conceal")
-  throw 'Skipped, conceal feature missing'
-endif
+source check.vim
+CheckOption linebreak
+CheckFeature conceal
 
 source view_util.vim