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.vim b/src/testdir/test_terminal.vim
index 09c050a..f1cd7da 100644
--- a/src/testdir/test_terminal.vim
+++ b/src/testdir/test_terminal.vim
@@ -1,8 +1,7 @@
 " Tests for the terminal window.
 
-if !has('terminal')
-  throw 'Skipped, terminal feature missing'
-endif
+source check.vim
+CheckFeature terminal
 
 source shared.vim
 source screendump.vim