Don't hard code server ports in SOCKS tests

Don't hard code server ports in SOCKS tests

* test/lisp/net/socks-tests.el (socks-tests-canned-server-create,
socks-tests-filter-response-parsing-v4): Fix bug in process filter to
prevent prepared outgoing responses from being implicitly encoded as
UTF-8.  Fix similar mistake in v4 filter test.
(socks-tests-v4-basic, socks-tests-v5-auth-user-pass,
socks-tests-v5-auth-user-pass-blank, socks-tests-v5-auth-none): Allow
system to choose port instead of hard-coding it.
(socks-tests-perform-hello-world-http-request): Add optional `method'
parameter to specify a gateway method.
(socks-tests-v5-auth-none): Move body to helper function of the same
name.
(socks-override-functions): New test ensuring top-level advice around
`open-networks-stream' still supported.  (Bug#53941)