pythonPackages.webassets: disable tests

Test invocation was broken. Now that is fixed, but getting the tests to
pass requires more work.

pkgs/top-level/python-packages.nix

@@ -26043,8 +26043,15 @@ in {
       sha256 = "14m13xa5sc7iqq2j1wsd2klcwaihqlhz2l9lmn92dks2yc8hplcr";
     };
 
+    buildInputs = with self; [ nose jinja2 mock pytest ];
     propagatedBuildInputs = with self; [ pyyaml ];
 
+    doCheck = false;
+
+    checkPhase = ''
+      py.test
+    '';
+
     meta = {
       description = "Media asset management for Python, with glue code for various web frameworks";
       homepage = "http://github.com/miracle2k/webassets/";