ipc: fail harder for abstract ipc on non-Linux

Using 'ipc://@abstract-socket' on non-Linux platforms yields inconsistent
behaviour.  Abstract sockets don't exist, so the literal file is created.
The test previously failed, but for a different reason: this is not the
directory you are looking for.  Now, zmq_bind() will fail for the right
reason: the socket can't be created.  Put the XFAIL back.