refactor: only use shaping dialer for ndt7 and dash (#754)

See https://github.com/ooni/probe/issues/2112 for context.

While there, run `go fix -fix buildtag ./...`

internal/netxlite/shaping_shaping.go

@@ -0,0 +1,43 @@
+//go:build shaping
+
+package netxlite
+
+import (
+	"context"
+	"net"
+	"time"
+
+	"github.com/ooni/probe-cli/v3/internal/model"
+)
+
+func newMaybeShapingDialer(dialer model.Dialer) model.Dialer {
+	return &shapingDialer{dialer}
+}
+
+type shapingDialer struct {
+	model.Dialer
+}
+
+// DialContext implements Dialer.DialContext
+func (d *shapingDialer) DialContext(
+	ctx context.Context, network, address string) (net.Conn, error) {
+	conn, err := d.Dialer.DialContext(ctx, network, address)
+	if err != nil {
+		return nil, err
+	}
+	return &shapingConn{Conn: conn}, nil
+}
+
+type shapingConn struct {
+	net.Conn
+}
+
+func (c *shapingConn) Read(p []byte) (int, error) {
+	time.Sleep(100 * time.Millisecond)
+	return c.Conn.Read(p)
+}
+
+func (c *shapingConn) Write(p []byte) (int, error) {
+	time.Sleep(100 * time.Millisecond)
+	return c.Conn.Write(p)
+}