lint: 'validate-uri' reports suspiciously small 200 responses.

lint: 'validate-uri' reports suspiciously small 200 responses.

* guix/scripts/lint.scm (validate-uri): Upon 200 http-response, check
the 'response-content-length' and emit a warning when it is <= 1000.
* tests/lint.scm (call-with-http-server): Add 'data' parameter.
(with-http-server): Likewise.
(%long-string): New variable.
("home-page: 200"): Pass %LONG-STRING to 'with-http-server'.
("home-page: 404", "source: 200", "source: 404"): Likewise.
("home-page: 200 but short length"): New test.
("source: 200 but short length"): New test.