patch 8.1.1108: test for 'visualbell' doesn't work

Problem:    Test for 'visualbell' doesn't work.
Solution:   Make 'belloff' empty.
diff --git a/src/testdir/test_options.vim b/src/testdir/test_options.vim
index 241c7b6..6cc0c1a 100644
--- a/src/testdir/test_options.vim
+++ b/src/testdir/test_options.vim
@@ -539,7 +539,9 @@
 endfunc
 
 func Test_visualbell()
+  set belloff=
   set visualbell
   call assert_beeps('normal 0h')
   set novisualbell
+  set belloff=all
 endfunc