tests: Adjust for 'http-fetch' change in (guix import json).

This is a followup to commit 81e0bc1834.

* tests/cpan.scm ("cpan->guix-package"): Add a 'rest' argument to the
lambda that mocks 'http-fetch'.
* tests/crate.scm ("crate->guix-package"): Likewise.
* tests/gem.scm ("gem->guix-package"): Likewise.
* tests/pypi.scm ("pypi->guix-package"): Likewise.
("pypi->guix-package, wheels"): Likewise.

tests/cpan.scm

@@ -72,7 +72,7 @@
                    test-source)
                   (_ (error "Unexpected URL: " url))))))))
         (mock ((guix http-client) http-fetch
-               (lambda (url)
+               (lambda (url . rest)
                  (match url
                    ("https://api.metacpan.org/release/Foo-Bar"
                     (values (open-input-string test-json)

tests/crate.scm

@@ -65,7 +65,7 @@
 (test-assert "crate->guix-package"
   ;; Replace network resources with sample data.
   (mock ((guix http-client) http-fetch
-         (lambda (url)
+         (lambda (url . rest)
            (match url
              ("https://crates.io/api/v1/crates/foo"
               (open-input-string test-crate))

tests/gem.scm

@@ -47,7 +47,7 @@
 (test-assert "gem->guix-package"
   ;; Replace network resources with sample data.
   (mock ((guix http-client) http-fetch
-         (lambda (url)
+         (lambda (url . rest)
            (match url
              ("https://rubygems.org/api/v1/gems/foo.json"
               (values (open-input-string test-json)

tests/pypi.scm

@@ -118,7 +118,7 @@ baz > 13.37")
                ("https://example.com/foo-1.0.0-py2.py3-none-any.whl" #f)
                (_ (error "Unexpected URL: " url)))))
           (mock ((guix http-client) http-fetch
-                 (lambda (url)
+                 (lambda (url . rest)
                    (match url
                      ("https://pypi.python.org/pypi/foo/json"
                       (values (open-input-string test-json)
@@ -181,7 +181,7 @@ baz > 13.37")
                  (delete-file-recursively "foo-1.0.0.dist-info")))
              (_ (error "Unexpected URL: " url)))))
         (mock ((guix http-client) http-fetch
-               (lambda (url)
+               (lambda (url . rest)
                  (match url
                    ("https://pypi.python.org/pypi/foo/json"
                     (values (open-input-string test-json)