refactor(ndt7): use netxlite rather than netx (#768)

This diff required us to move some code around, but no major
change actually happened, except better tests.

While there, I also slightly refactored ndt7's implementation and
removed the ProxyURL setting, which was actually unused.

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

internal/engine/netx/dialer/bytecounter.go

@@ -1,26 +1,5 @@
 package dialer
 
-import (
-	"context"
-	"net"
+import "github.com/ooni/probe-cli/v3/internal/bytecounter"
 
-	"github.com/ooni/probe-cli/v3/internal/bytecounter"
-	"github.com/ooni/probe-cli/v3/internal/model"
-)
-
-// byteCounterDialer is a byte-counting-aware dialer. To perform byte counting, you
-// should make sure that you insert this dialer in the dialing chain.
-type byteCounterDialer struct {
-	model.Dialer
-}
-
-// DialContext implements Dialer.DialContext
-func (d *byteCounterDialer) DialContext(
-	ctx context.Context, network, address string) (net.Conn, error) {
-	conn, err := d.Dialer.DialContext(ctx, network, address)
-	if err != nil {
-		return nil, err
-	}
-	conn = bytecounter.MaybeWrapWithContextByteCounters(ctx, conn)
-	return conn, nil
-}
+type byteCounterDialer = bytecounter.ContextAwareDialer