patch 8.2.0905: test coverage could be better

Problem:    Test coverage could be better.
Solution:   Add a couple of tests. (Dominique Pelle, closes #6202)
diff --git a/src/testdir/test_cmdline.vim b/src/testdir/test_cmdline.vim
index a547326..66f6e90 100644
--- a/src/testdir/test_cmdline.vim
+++ b/src/testdir/test_cmdline.vim
@@ -1220,6 +1220,22 @@
   call delete('Xresult')
 endfunc
 
+" Test error: "E135: *Filter* Autocommands must not change current buffer"
+func Test_cmd_bang_E135()
+  new
+  call setline(1, ['a', 'b', 'c', 'd'])
+  augroup test_cmd_filter_E135
+    au!
+    autocmd FilterReadPost * help
+  augroup END
+  call assert_fails('2,3!echo "x"', 'E135:')
+
+  augroup test_cmd_filter_E135
+    au!
+  augroup END
+  %bwipe!
+endfunc
+
 " Test for using ~ for home directory in cmdline completion matches
 func Test_cmdline_expand_home()
   call mkdir('Xdir')