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_xxd.vim b/src/testdir/test_xxd.vim
index 63ac574..cf47c8a 100644
--- a/src/testdir/test_xxd.vim
+++ b/src/testdir/test_xxd.vim
@@ -2,7 +2,7 @@
 if empty($XXD) && executable('..\xxd\xxd.exe')
   let s:xxd_cmd = '..\xxd\xxd.exe'
 elseif empty($XXD) || !executable($XXD)
-  throw 'Skipped, xxd program missing'
+  throw 'Skipped: xxd program missing'
 else
   let s:xxd_cmd = $XXD
 endif