Race condition in 04_reconnect.t

[nponeccop]

Some CPAN testers demonstrate a race condition in 04_reconnect.t. Somehow on my development VM it happens every time:

http://www.cpantesters.org/cpan/report/0d4eba30-2867-11e7-8188-e43c188d0c79

Basically connection from AnyEvent::Gearman::Client is rejected earlier than the server part starts.

The fix I use (I'll make a PR later) is to put a wait_port call before we attempt a connection.