patch 8.0.1224: still interference between test functions

Problem:    Still interference between test functions.
Solution:   Clear autocommands. Wipe all buffers.  Fix tests that depend on a
            specific start context.
diff --git a/src/testdir/test_autocmd.vim b/src/testdir/test_autocmd.vim
index aa7d157..dbf7d20 100644
--- a/src/testdir/test_autocmd.vim
+++ b/src/testdir/test_autocmd.vim
@@ -254,16 +254,15 @@
   au BufReadCmd * e +h
   help
 
-  helpclose
   au! BufReadCmd
 endfunc
 
 func Test_BufReadCmdHelpJump()
   " This used to cause access to free memory
   au BufReadCmd * e +h{
-  help
+  " } to fix highlighting
+  call assert_fails('help', 'E434:')
 
-  helpclose
   au! BufReadCmd
 endfunc