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_sha256.vim b/src/testdir/test_sha256.vim
index 7f802b1..d0f92b1 100644
--- a/src/testdir/test_sha256.vim
+++ b/src/testdir/test_sha256.vim
@@ -1,11 +1,8 @@
 " Tests for the sha256() function.
 
-if !has('cryptv')
-  throw 'Skipped, cryptv feature missing'
-endif
-if !exists('*sha256')
-  throw 'Skipped, sha256 function missing'
-endif
+source check.vim
+CheckFeature cryptv
+CheckFunction sha256
 
 function Test_sha256()
   " test for empty string: