feat: reintroduce the tproxy functionality (#975)

We originally removed the TProxy in https://github.com/ooni/probe/issues/2224. Nevertheless, in https://github.com/ooni/probe-cli/pull/969, we determined that something like the previous TProxy, with small changes, was required to support https://github.com/ooni/probe/issues/2340. So, this pull request reintroduces a slightly-modified TProxy functionality that better adapts to the `--remote=REMOTE` use case.

internal/netxlite/tproxy_test.go

@@ -0,0 +1,22 @@
+package netxlite
+
+import (
+	"context"
+	"strings"
+	"testing"
+	"time"
+)
+
+func TestDefaultTProxy(t *testing.T) {
+	t.Run("DialContext honours the timeout", func(t *testing.T) {
+		tp := &DefaultTProxy{}
+		ctx := context.Background()
+		conn, err := tp.DialContext(ctx, 100*time.Microsecond, "tcp", "1.1.1.1:443")
+		if err == nil || !strings.HasSuffix(err.Error(), "i/o timeout") {
+			t.Fatal(err)
+		}
+		if conn != nil {
+			t.Fatal("expected nil conn")
+		}
+	})
+}