patch 8.0.0433: beeps when running tests

Problem:    Quite a few beeps when running tests.
Solution:   Set 'belloff' for these tests. (Christian Brabandt)
diff --git a/src/testdir/test_packadd.vim b/src/testdir/test_packadd.vim
index 8ffebf9..9d4c6d8 100644
--- a/src/testdir/test_packadd.vim
+++ b/src/testdir/test_packadd.vim
@@ -1,5 +1,7 @@
 " Tests for 'packpath' and :packadd
 
+set belloff=all
+
 func SetUp()
   let s:topdir = expand('%:h') . '/Xdir'
   exe 'set packpath=' . s:topdir