The server previously responded with a copy of the requested "Host" header in
the response, without any validation. That header, as defined in RFC 9110, is
intended for use in *requests*; remove it from the responses.
Port the main php_cli_server.inc to use ephemeral ports, thus
allowing CLI server tests to be parallelized.
A complication here is that we also need to give each test a
separate doc root, to avoid index.php files writing over each
other.
Closes GH-6375.
Daniel Scherzer