Wait for WEBrick server start during tests

[marcotc]

Fixes this flaky test:

  1) Adapters::UnixSocket integration tests when sending traces through Unix socket client sends traces successfully
     Failure/Error: expect(messages).to have(1).items
       expected 1 items, got 0
     # ./spec/ddtrace/transport/http/adapters/unix_socket_integration_spec.rb:79:in `block (3 levels) in <top (required)>'

This happens because we don't wait until WEBrick::HTTPServer#start finishes its internal socket initialization before trying to use it.

Unfortunately, WEBrick::HTTPServer#start is a method that never returns. We need to use a special callback parameter, StartCallback, to inform us that the server is ready.

[delner]

👍