test/old: Test_BufReadCmdHelp: generate helptags

Generate helptags before :help.

This only failed in the TSAN build because other builds run the Lua
tests first, which generate the helptags.

closes #8824

src/nvim/testdir/test_autocmd.vim

@@ -251,6 +251,7 @@ func Test_augroup_warning()
 endfunc
 
 func Test_BufReadCmdHelp()
+  helptags ALL
   " This used to cause access to free memory
   au BufReadCmd * e +h
   help