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_usercommands.vim b/src/testdir/test_usercommands.vim
index 9420321..29db51f 100644
--- a/src/testdir/test_usercommands.vim
+++ b/src/testdir/test_usercommands.vim
@@ -1,6 +1,7 @@
 " Tests for user defined commands
 
 " Test for <mods> in user defined commands
+set belloff=all
 function Test_cmdmods()
   let g:mods = ''