refactor(tracex): convert to unit testing (#781)

The exercise already allowed me to notice issues such as fields not
being properly initialized by savers.

This is one of the last steps before moving tracex away from the
internal/netx package and into the internal package.

See https://github.com/ooni/probe/issues/2121

internal/engine/netx/tracex/http.go

@@ -27,11 +27,17 @@ func httpCloneRequestHeaders(req *http.Request) http.Header {
 	return header
 }
 
-// SaverTransactionHTTPTransport is a RoundTripper that saves
+// HTTPTransportSaver is a RoundTripper that saves
 // events related to the HTTP transaction
-type SaverTransactionHTTPTransport struct {
-	model.HTTPTransport
-	Saver        *Saver
+type HTTPTransportSaver struct {
+	// HTTPTransport is the MANDATORY underlying HTTP transport.
+	HTTPTransport model.HTTPTransport
+
+	// Saver is the MANDATORY saver to use.
+	Saver *Saver
+
+	// SnapshotSize is the OPTIONAL maximum body snapshot size (if not set, we'll
+	// use 1<<17, which we've been using since the ooni/netx days)
 	SnapshotSize int64
 }
 
@@ -40,7 +46,11 @@ type SaverTransactionHTTPTransport struct {
 //
 // The maxBodySnapshotSize argument controls the maximum size of the
 // body snapshot that we collect along with the HTTP round trip.
-func (txp *SaverTransactionHTTPTransport) RoundTrip(req *http.Request) (*http.Response, error) {
+func (txp *HTTPTransportSaver) RoundTrip(req *http.Request) (*http.Response, error) {
+
+	// TODO(bassosimone): we're currently using the started time for
+	// the transaction done event, which contrasts with what we do for
+	// every other event. What does the spec say?
 
 	started := time.Now()
 	txp.Saver.Write(&EventHTTPTransactionStart{&EventValue{
@@ -92,7 +102,15 @@ func (txp *SaverTransactionHTTPTransport) RoundTrip(req *http.Request) (*http.Re
 	return resp, nil
 }
 
-func (txp *SaverTransactionHTTPTransport) snapshotSize() int64 {
+func (txp *HTTPTransportSaver) CloseIdleConnections() {
+	txp.HTTPTransport.CloseIdleConnections()
+}
+
+func (txp *HTTPTransportSaver) Network() string {
+	return txp.HTTPTransport.Network()
+}
+
+func (txp *HTTPTransportSaver) snapshotSize() int64 {
 	if txp.SnapshotSize > 0 {
 		return txp.SnapshotSize
 	}
@@ -104,4 +122,4 @@ type httpReadableAgainBody struct {
 	io.Closer
 }
 
-var _ model.HTTPTransport = &SaverTransactionHTTPTransport{}
+var _ model.HTTPTransport = &HTTPTransportSaver{}