Start test servers on a port the system picks (closes #211)

The internal/server and internal/ircserver tests picked a port by
opening a listener, reading its port and closing it. They then started
a server on that number. Another process could take the port in
between, and a test failed with "address already in use".

The IRC server tests now start the server on 127.0.0.1:0 and read the
port back from its listener. The HTTP server tests start the app with
port 0. The HTTP server now keeps the listener it binds, so the stop
test sends its long poll to the port the app was given.

Model: opus-5-5

internal/server/export_test.go

@@ -15,6 +15,12 @@ func NewTestSPAServer() *Server {
 	return srv
 }
 
+// Listener returns the listener the start hook bound for the
+// HTTP server.
+func (srv *Server) Listener() net.Listener {
+	return srv.listener
+}
+
 // ServeUntilShutdown runs serveUntilShutdown, which serves the
 // HTTP server on listener, and returns when it does.
 func (srv *Server) ServeUntilShutdown(listener net.Listener) {