Integration tests fail with 'Errno::EADDRINUSE'

[martin-schmidt]

All tests in spec/integration/version_handshake_spec.rb bind BN::P2p::Server to port 3333. It's not ensured that the socket opened by a previous test is already closed when the next test starts.

This leads to an error:
Errno::EADDRINUSE: Address already in use - bind(2) for "localhost" port 3333