tests: Support multiple HTTP server instances.
* guix/tests/http.scm (%http-server-socket): Turn into...
(open-http-server-socket): ... this procedure.
(http-server-can-listen?): New procedure.
(http-write, %http-server-lock, %http-server-ready)
(http-open, stub-http-server): Move to 'call-with-http-server' body.
(call-with-http-server): Add #:headers parameter.
(with-http-server): Add an additional pattern with headers.
* tests/derivations.scm: Use (http-server-can-listen?) instead
of (force %http-server-socket).
* tests/lint.scm: Likewise.
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