refactor(netx): reorganize by topic (#800)

Before finishing the ongoing refactoring and leaving whatever
is left of netx in tree, I would like to restructure it so that
we'll have an easy time next time we need to modify it.

Currently, every functionality lives into the `netx.go` file and
we have a support file called `httptransport.go`.

I would like to reorganize by topic, instead. This would allow
future me to more easily perform topic-specific changes.

While there, improve `netx`'s documentation and duplicate some of
this documentation inside `internal/README.md` to provide pointers
to previous documentation, historical context, and some help to
understand the logic architecture of network extensions (aka `netx`).

Part of https://github.com/ooni/probe-cli/pull/396

internal/engine/netx/tls_test.go

@@ -0,0 +1,74 @@
+package netx
+
+import (
+	"context"
+	"crypto/tls"
+	"testing"
+
+	"github.com/ooni/probe-cli/v3/internal/netxlite"
+	"github.com/ooni/probe-cli/v3/internal/netxlite/filtering"
+	"github.com/ooni/probe-cli/v3/internal/tracex"
+)
+
+func TestNewTLSDialer(t *testing.T) {
+	t.Run("we always have error wrapping", func(t *testing.T) {
+		server := filtering.NewTLSServer(filtering.TLSActionReset)
+		defer server.Close()
+		tdx := NewTLSDialer(Config{})
+		conn, err := tdx.DialTLSContext(context.Background(), "tcp", server.Endpoint())
+		if err == nil || err.Error() != netxlite.FailureConnectionReset {
+			t.Fatal("unexpected err", err)
+		}
+		if conn != nil {
+			t.Fatal("expected nil conn")
+		}
+	})
+
+	t.Run("we can collect measurements", func(t *testing.T) {
+		server := filtering.NewTLSServer(filtering.TLSActionReset)
+		defer server.Close()
+		saver := &tracex.Saver{}
+		tdx := NewTLSDialer(Config{
+			Saver: saver,
+		})
+		conn, err := tdx.DialTLSContext(context.Background(), "tcp", server.Endpoint())
+		if err == nil || err.Error() != netxlite.FailureConnectionReset {
+			t.Fatal("unexpected err", err)
+		}
+		if conn != nil {
+			t.Fatal("expected nil conn")
+		}
+		if len(saver.Read()) <= 0 {
+			t.Fatal("did not read any event")
+		}
+	})
+
+	t.Run("we can skip TLS verification", func(t *testing.T) {
+		server := filtering.NewTLSServer(filtering.TLSActionBlockText)
+		defer server.Close()
+		tdx := NewTLSDialer(Config{TLSConfig: &tls.Config{
+			InsecureSkipVerify: true,
+		}})
+		conn, err := tdx.DialTLSContext(context.Background(), "tcp", server.Endpoint())
+		if err != nil {
+			t.Fatal(err.(*netxlite.ErrWrapper).WrappedErr)
+		}
+		conn.Close()
+	})
+
+	t.Run("we can set the cert pool", func(t *testing.T) {
+		server := filtering.NewTLSServer(filtering.TLSActionBlockText)
+		defer server.Close()
+		tdx := NewTLSDialer(Config{
+			TLSConfig: &tls.Config{
+				RootCAs:    server.CertPool(),
+				ServerName: "dns.google",
+			},
+		})
+		conn, err := tdx.DialTLSContext(context.Background(), "tcp", server.Endpoint())
+		if err != nil {
+			t.Fatal(err)
+		}
+		conn.Close()
+	})
+}