tests: Mock up http-fetch in import test.

This is a follow-up to commit 63773200d7.

* tests/pypi.scm ("pypi->guix-package, wheels"): Add mock definition of
"http-fetch".

tests/pypi.scm

@@ -148,10 +148,6 @@ baz > 13.37")
   (mock ((guix import utils) url-fetch
          (lambda (url file-name)
            (match url
-             ("https://pypi.python.org/pypi/foo/json"
-              (with-output-to-file file-name
-                (lambda ()
-                  (display test-json))))
              ("https://example.com/foo-1.0.0.tar.gz"
                (begin
                  (mkdir "foo-1.0.0")
@@ -175,6 +171,14 @@ baz > 13.37")
                    (rename-file zip-file file-name))
                  (delete-file-recursively "foo-1.0.0.dist-info")))
              (_ (error "Unexpected URL: " url)))))
+        (mock ((guix http-client) http-fetch
+               (lambda (url)
+                 (match url
+                   ("https://pypi.python.org/pypi/foo/json"
+                    (values (open-input-string test-json)
+                            (string-length test-json)))
+                   ("https://example.com/foo-1.0.0-py2.py3-none-any.whl" #f)
+                   (_ (error "Unexpected URL: " url)))))
               (match (pypi->guix-package "foo")
                 (('package
                    ('name "python-foo")
@@ -200,6 +204,6 @@ baz > 13.37")
                             test-source-hash)
                            hash))
                 (x
-       (pk 'fail x #f)))))
+                 (pk 'fail x #f))))))
 
 (test-end "pypi")