gnu: nss: Disable long b64 tests.

Suggested by Marius Bakke <mbakke@fastmail.com> in
<https://lists.gnu.org/archive/html/guix-devel/2017-04/msg00440.html>.

* gnu/packages/patches/nss-disable-long-b64-tests.patch: New file.
* gnu/local.mk (dist_patch_DATA): Add it.
* gnu/packages/gnuzilla.scm (nss)[source]: Add patch.