* guix/import/texlive.scm (fetch-sxml, sxml->package): Export.
* tests/texlive.scm <top level>: Call '%http-server-port'.
("fetch-sxml: returns SXML for valid XML"): Use 'with-http-server' and
set 'current-http-proxy' instead of using 'mock'.
("sxml->package"): Remove use of '@@'.