patch 8.0.0485: not all windows commands are tested

Problem:    Not all windows commands are tested.
Solution:   Add more tests for windows commands. (Dominique Pelle,
            closes #1575) Run test_autocmd separately, it interferes with
            other tests.  Fix tests that depended on side effects.
diff --git a/src/testdir/test_autocmd.vim b/src/testdir/test_autocmd.vim
index 566a07c..df6a868 100644
--- a/src/testdir/test_autocmd.vim
+++ b/src/testdir/test_autocmd.vim
@@ -1,5 +1,7 @@
 " Tests for autocommands
 
+set belloff=all
+
 function! s:cleanup_buffers() abort
   for bnr in range(1, bufnr('$'))
     if bufloaded(bnr) && bufnr('%') != bnr
@@ -318,6 +320,8 @@
   call assert_equal('Xanother', expand('%'))
 
   au!
+  enew
+  bwipe! Xtestje1
   call delete('Xtestje1')
   call delete('Xtestje2')
   call delete('Xtestje3')