feat: port-filtering experiment (#891)

Part of https://github.com/ooni/probe/issues/2005

internal/cmd/ooporthelper/main_test.go

@@ -0,0 +1,31 @@
+package main
+
+import (
+	"context"
+	"net"
+	"testing"
+
+	"github.com/ooni/probe-cli/v3/internal/model"
+	"github.com/ooni/probe-cli/v3/internal/netxlite"
+)
+
+func TestMainWorkingAsIntended(t *testing.T) {
+	srvTest = true // toggle to imply that we are running in test mode
+	go main()
+	dialer := netxlite.NewDialerWithoutResolver(model.DiscardLogger)
+	for _, port := range TestPorts {
+		<-srvTestChan
+		addr := net.JoinHostPort("127.0.0.1", port)
+		ctx := context.Background()
+		conn, err := dialer.DialContext(ctx, "tcp", addr)
+		if err != nil {
+			t.Fatal(err)
+		}
+		if conn == nil {
+			t.Fatal("expected non-nil conn")
+		}
+		conn.Close()
+	}
+	srvCancel()  // shutdown server
+	srvWg.Wait() // wait for listeners on all ports to close
+}