refactor(netxlite): improve tests for http and http3 (#487)

* refactor(netxlite): improve tests for http and http3

See https://github.com/ooni/probe/issues/1591

* Update internal/netxlite/http3.go

internal/netxlite/mocks/http3.go

@@ -0,0 +1,19 @@
+package mocks
+
+import "net/http"
+
+// HTTP3RoundTripper allows mocking http3.RoundTripper.
+type HTTP3RoundTripper struct {
+	MockRoundTrip func(req *http.Request) (*http.Response, error)
+	MockClose     func() error
+}
+
+// RoundTrip calls MockRoundTrip.
+func (txp *HTTP3RoundTripper) RoundTrip(req *http.Request) (*http.Response, error) {
+	return txp.MockRoundTrip(req)
+}
+
+// Close calls MockClose.
+func (txp *HTTP3RoundTripper) Close() error {
+	return txp.MockClose()
+}

internal/netxlite/mocks/http3_test.go

@@ -0,0 +1,37 @@
+package mocks
+
+import (
+	"errors"
+	"net/http"
+	"testing"
+)
+
+func TestHTTP3RoundTripper(t *testing.T) {
+	t.Run("RoundTrip", func(t *testing.T) {
+		expected := errors.New("mocked error")
+		txp := &HTTP3RoundTripper{
+			MockRoundTrip: func(req *http.Request) (*http.Response, error) {
+				return nil, expected
+			},
+		}
+		resp, err := txp.RoundTrip(&http.Request{})
+		if !errors.Is(err, expected) {
+			t.Fatal("unexpected err", err)
+		}
+		if resp != nil {
+			t.Fatal("unexpected resp")
+		}
+	})
+
+	t.Run("Close", func(t *testing.T) {
+		expected := errors.New("mocked error")
+		txp := &HTTP3RoundTripper{
+			MockClose: func() error {
+				return expected
+			},
+		}
+		if err := txp.Close(); !errors.Is(err, expected) {
+			t.Fatal("unexpected err", err)
+		}
+	})
+}