Connection: add explicit $self->{connection}->close because otherwise the connection would persist because the responder might keep the socket. Pass the Echo test with Twiggy.
t/testlib/Echo: some refactoring. Add tests about UTF-8 encoded text.
Add t/testlib/Cycle.pm. Add t/testlib/Master.pm to gather test modules and run them as subtests. t/twiggy.t runs testlib::Master::run_tests with Twiggy::Server.
WebSocket: rename on_fallback -> on_error, because the callback is rather an error handler when the request is invalid.
Add t/testlib/Responder.pm. Plack::App::WebSocket::Connection::WAIT_FOR_FLUSHING_SEC is now a package variable, which is configured during the Responder test. The variable is hidden from the POD, though.