Fixes a regression introduced in
fc29c80b96, where, since
'get-opam-repository' was no longer mocked, the test would try to access
the actual OPAM repository through a call to 'http-fetch/cached'; this
would lead to a test failure when networking is unavailable.
* tests/opam.scm ("opam->guix-package"): Mock 'get-opam-repository'
again.