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_matchadd_conceal_utf8.vim b/src/testdir/test_matchadd_conceal_utf8.vim
index fb27577..98ed2cb 100644
--- a/src/testdir/test_matchadd_conceal_utf8.vim
+++ b/src/testdir/test_matchadd_conceal_utf8.vim
@@ -1,8 +1,7 @@
 " Test for matchadd() and conceal feature using utf-8.
 
-if !has('conceal')
-  throw 'Skipped, conceal feature missing'
-endif
+source check.vim
+CheckFeature conceal
 
 if !has('gui_running') && has('unix')
   set term=ansi