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_cmdline.vim b/src/testdir/test_cmdline.vim
index 8d5ae28..98a4337 100644
--- a/src/testdir/test_cmdline.vim
+++ b/src/testdir/test_cmdline.vim
@@ -1,5 +1,7 @@
 " Tests for editing the command line.
 
+set belloff=all
+
 func Test_complete_tab()
   call writefile(['testfile'], 'Xtestfile')
   call feedkeys(":e Xtest\t\r", "tx")