import: opam: Avoid uses of '@@' in tests.

import: opam: Avoid uses of '@@' in tests.

* guix/import/opam.scm (string-pat, multiline-string, list-pat)
(dict, condition): Export.
(opam-fetch): Add optional 'repository' parameter.
(opam->guix-package): Add #:repository parameter and pass it to
'opam-fetch'.
* tests/opam.scm ("opam->guix-package"): Remove use of 'mock' and pass
TEST-REPO to 'opam->guix-package' instead.
("parse-strings", "parse-multiline-strings")
("parse-lists", "parse-dicts", "parse-conditions"): Remove uses of '@@',
which are no longer needed.