vim-patch:8.0.0433: beeps when running tests

Problem:    Quite a few beeps when running tests.
Solution:   Set 'belloff' for these tests. (Christian Brabandt)

c3c766ea8c

src/nvim/testdir/test_undo.vim

@@ -3,6 +3,7 @@
 " undo-able pieces.  Do that by setting 'undolevels'.
 " Also tests :earlier and :later.
 
+set belloff=all
 func Test_undotree()
   exe "normal Aabc\<Esc>"
   set ul=100