refactor(netx): remove the self censorship mechanism (#364)

We're currently use jafar for QA and jafar is a better mechanism,
even though it is not portable outside of Linux.

This self censorship mechanism was less cool and added a bunch
of (also cognitive) complexity to netx.

If we ever want to go down a self censorship like road, we probably
want to do as little work as possible in the problem and as much
work as possible inside a helper like jafar.

Part of https://github.com/ooni/probe/issues/1591.

internal/engine/netx/dialer/system.go

@@ -0,0 +1,24 @@
+package dialer
+
+import (
+	"context"
+	"net"
+	"time"
+)
+
+// defaultNetDialer is the net.Dialer we use by default.
+var defaultNetDialer = &net.Dialer{
+	Timeout:   15 * time.Second,
+	KeepAlive: 15 * time.Second,
+}
+
+// SystemDialer is the system dialer.
+type SystemDialer struct{}
+
+// DialContext implements Dialer.DialContext
+func (d SystemDialer) DialContext(ctx context.Context, network, address string) (net.Conn, error) {
+	return defaultNetDialer.DialContext(ctx, network, address)
+}
+
+// Default is the dialer we use by default.
+var Default = SystemDialer{}

internal/engine/netx/dialer/system_test.go

@@ -0,0 +1,20 @@
+package dialer
+
+import (
+	"strings"
+	"testing"
+
+	"github.com/ooni/psiphon/oopsi/golang.org/x/net/context"
+)
+
+func TestSystemDialer(t *testing.T) {
+	ctx, cancel := context.WithCancel(context.Background())
+	cancel() // fail immediately
+	conn, err := Default.DialContext(ctx, "tcp", "8.8.8.8:853")
+	if err == nil || !strings.HasSuffix(err.Error(), "operation was canceled") {
+		t.Fatal("not the error we expected", err)
+	}
+	if conn != nil {
+		t.Fatal("expected nil conn here")
+	}
+}