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_erasebackword.in b/src/testdir/test_erasebackword.in
index 1ddcd3c..36dd232 100644
--- a/src/testdir/test_erasebackword.in
+++ b/src/testdir/test_erasebackword.in
@@ -2,6 +2,7 @@
 
 STARTTEST
 :so small.vim
+:set belloff=all
 :so mbyte.vim
 :set encoding=utf-8
 G