tests: 'with-http-server' accepts multiple responses.

tests: 'with-http-server' accepts multiple responses.

* guix/tests/http.scm (call-with-http-server): Replace 'code' and 'data'
parameters with 'responses+data'.  Compute RESPONSES as a function of
that.  Remove #:headers parameter.
[http-write]: Quit only when RESPONSES is empty.
[server-body]: Get the response and data from RESPONSES, and set it to
point to the rest.
(with-http-server): Adjust accordingly.
* tests/derivations.scm ("'download' built-in builder")
("'download' built-in builder, invalid hash")
("'download' built-in builder, not found")
("'download' built-in builder, check mode"): Adjust to new
'with-http-server' interface.
* tests/lint.scm ("home-page: 200")
("home-page: 200 but short length")
("home-page: 404", "home-page: 301, invalid"):
("home-page: 301 -> 200", "home-page: 301 -> 404")
("source: 200", "source: 200 but short length")
("source: 404", "source: 404 and 200")
("source: 301 -> 200", "source: 301 -> 404"):
("github-url", github-url): Likewise.
* tests/swh.scm (with-json-result)
("lookup-origin, not found"): Likewise.