gnu: python-cffi: Really enable tests.

* gnu/packages/python.scm (python-cffi, python2-cffi)[arguments]: Replace
'check' phase with custom expression. Change to 'modify-phases' syntax.

gnu/packages/python.scm

@@ -4781,9 +4781,21 @@ a front-end for C compilers or analysis tools.")
        ("python-sphinx" ,python-sphinx)
        ("python-pytest" ,python-pytest)))
     (arguments
-     `(#:phases
-       (alist-cons-after
-        'install 'install-doc
+     `(#:modules ((ice-9 ftw)
+                  (srfi srfi-26)
+                  (guix build utils)
+                  (guix build python-build-system))
+       #:phases
+       (modify-phases %standard-phases
+         (replace 'check
+           (lambda _
+             (setenv "PYTHONPATH"
+                     (string-append
+                      (getenv "PYTHONPATH")
+                      ":" (getcwd) "/build/"
+                      (car (scandir "build" (cut string-prefix? "lib." <>)))))
+             (zero? (system* "py.test" "-v"))))
+         (add-after 'install 'install-doc
            (lambda* (#:key outputs #:allow-other-keys)
              (let* ((data (string-append (assoc-ref outputs "doc") "/share"))
                     (doc (string-append data "/doc/" ,name "-" ,version))
@@ -4792,8 +4804,8 @@ a front-end for C compilers or analysis tools.")
                  (system* "make" "html")
                  (mkdir-p html)
                  (copy-recursively "build/html" html))
-            (copy-file "LICENSE" (string-append doc "/LICENSE"))))
-        %standard-phases)))
+               (copy-file "LICENSE" (string-append doc "/LICENSE"))
+               #t))))))
     (home-page "http://cffi.readthedocs.org")
     (synopsis "Foreign function interface for Python")
     (description