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_terminal_fail.vim b/src/testdir/test_terminal_fail.vim
index d010c3b..f33ade6 100644
--- a/src/testdir/test_terminal_fail.vim
+++ b/src/testdir/test_terminal_fail.vim
@@ -2,9 +2,8 @@
 " leaks under valgrind.  That is because when fork/exec fails memory is not
 " freed.  Since the process exists right away it's not a real leak.
 
-if !has('terminal')
-  throw 'Skipped, terminal feature missing'
-endif
+source check.vim
+CheckFeature terminal
 
 source shared.vim