feat: add a simple dnsping experiment (#674)

See https://github.com/ooni/probe/issues/1987 (issue).

See https://github.com/ooni/spec/pull/238 (impl).

While there, fix the build for go1.18 by adding go1.18 specific tests. I was
increasingly bothered by the build being red.

internal/engine/experiment/simplequicping/simplequicping_test.go

@@ -98,11 +98,11 @@ func TestMeasurer_run(t *testing.T) {
 	})
 
 	t.Run("with local listener", func(t *testing.T) {
-		srvrURL, err := startEchoServer()
+		srvrURL, listener, err := startEchoServer()
 		if err != nil {
 			log.Fatal(err)
 		}
-		t.Log(srvrURL)
+		defer listener.Close()
 		meas, m, err := runHelper(srvrURL)
 		if err != nil {
 			t.Fatal(err)
@@ -127,10 +127,10 @@ func TestMeasurer_run(t *testing.T) {
 // SPDX-License-Identifier: MIT
 //
 // See https://github.com/lucas-clemente/quic-go/blob/v0.27.0/example/echo/echo.go#L34
-func startEchoServer() (string, error) {
+func startEchoServer() (string, quic.Listener, error) {
 	listener, err := quic.ListenAddr("127.0.0.1:0", generateTLSConfig(), nil)
 	if err != nil {
-		return "", err
+		return "", nil, err
 	}
 	go echoWorkerMain(listener)
 	URL := &url.URL{
@@ -138,7 +138,7 @@ func startEchoServer() (string, error) {
 		Host:   listener.Addr().String(),
 		Path:   "/",
 	}
-	return URL.String(), nil
+	return URL.String(), listener, nil
 }
 
 // Worker used by startEchoServer to accept a quic connection.
@@ -147,16 +147,17 @@ func startEchoServer() (string, error) {
 //
 // See https://github.com/lucas-clemente/quic-go/blob/v0.27.0/example/echo/echo.go#L34
 func echoWorkerMain(listener quic.Listener) {
-	defer listener.Close()
-	conn, err := listener.Accept(context.Background())
-	if err != nil {
-		panic(err)
+	for {
+		conn, err := listener.Accept(context.Background())
+		if err != nil {
+			return
+		}
+		stream, err := conn.AcceptStream(context.Background())
+		if err != nil {
+			continue
+		}
+		stream.Close()
 	}
-	stream, err := conn.AcceptStream(context.Background())
-	if err != nil {
-		panic(err)
-	}
-	stream.Close()
 }
 
 // Setup a bare-bones TLS config for the server.