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_digraph.vim b/src/testdir/test_digraph.vim
index 92d4e47..6435c86 100644
--- a/src/testdir/test_digraph.vim
+++ b/src/testdir/test_digraph.vim
@@ -1,8 +1,7 @@
 " Tests for digraphs
 
-if !has("digraphs")
-  throw 'Skipped, digraphs feature missing'
-endif
+source check.vim
+CheckFeature digraphs
 
 func Put_Dig(chars)
   exe "norm! o\<c-k>".a:chars