import: elpa: Rewrite test to use an HTTP server instead of mocking.

* guix/import/elpa.scm (elpa-url): Add 'gnu/http'.
(elpa->guix-package): Handle it.
* tests/elpa.scm (elpa-package-info-mock, auctex-readme-mock)
(elpa-version->string, package-source-url, ensure-list)
(package-home-page, make-elpa-package): Remove.
<top level>: Call '%http-server-port'.
(eval-test-with-elpa): Remove uses of 'mock'.  Use 'with-http-server'
and parameterize 'current-http-proxy' instead.