ISPN-2503 Re-enable FD_SOCK in the test suite

https://issues.jboss.org/browse/ISPN-2503

Please cherry-pick on `5.2.x` as well.

Fixes the core test suite hanging on Sanne's machine.

It does create 2 extra threads per cache manager (1 pinger + 1 acceptor), but none of them will actually do any work.