Implement NTLM server for ntlm.el testing

Implement NTLM server for ntlm.el testing

* test/Makefile.in
(GNU_ELPA_DIRECTORY, elpa_dependencies, elpa_els, elpa_opts): New
variables.
(EMACSOPT, ert_opts): Add elpa_opts.
* test/README: Document GNU_ELPA_DIRECTORY make variable.
* test/lisp/net/ntlm-tests.el: Fix checkdoc-reported issues.
(ntlm-tests-message, ntlm-server-build-type-2, ntlm-server-hash)
(ntlm-server-check-authorization, ntlm-server-do-token)
(ntlm-server-filter, ntlm-server-handler, ntlm-server-start)
(ntlm-server-stop, ntlm-tests--url-retrieve-internal-around)
(ntlm-tests--authenticate)
(ntlm-tests--start-server-authenticate-stop-server): New
functions.
(ntlm-tests--username-oem, ntlm-tests--username-unicode)
(ntlm-tests--client-supports-unicode, ntlm-tests--challenge)
(ntlm-tests--result-buffer, ntlm-tests--successful-result): New
variables.
(ntlm-authentication)
(ntlm-authentication-old-compatibility-level): New tests.
* test/lisp/net/ntlm-resources/authinfo: New file.  (Bug#43566)