Remove test running from python-billiard

Fedora 21 hits https://bugzilla.redhat.com/show_bug.cgi?id=1161166 during test
execution. Since we are using a tarball directly from upstream, it is safer to
disable the tests than to patch python-billiard with the workaround mentioned
in the bz.
Merge pull request #1453 from beav/disable-billiard-tests

Remove test running from python-billiard