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_crypt.vim b/src/testdir/test_crypt.vim
index db10099..ab62d84 100644
--- a/src/testdir/test_crypt.vim
+++ b/src/testdir/test_crypt.vim
@@ -1,8 +1,7 @@
 " Tests for encryption.
 
-if !has('cryptv')
-  throw 'Skipped, encryption feature missing'
-endif
+source check.vim
+CheckFeature cryptv
 
 func Common_head_only(text)
   " This was crashing Vim