refactor(netx): move construction logic outside package (#798)

For testability, replace most if-based construction logic with
calls to well-tested factories living in other packages.

While there, acknowledge that a bunch of types could now be private
and make them private, modifying the code to call the public
factories allowing to construct said types instead.

Part of https://github.com/ooni/probe/issues/2121

internal/netxlite/bogon.go

@@ -14,6 +14,16 @@ import (
 	"github.com/ooni/probe-cli/v3/internal/runtimex"
 )
 
+// MaybeWrapWithBogonResolver wraps the given resolver with a BogonResolver
+// iff the provided boolean flag is true. Otherwise, this factory just returns
+// the provided resolver to the caller without any wrapping.
+func MaybeWrapWithBogonResolver(enabled bool, reso model.Resolver) model.Resolver {
+	if enabled {
+		reso = &BogonResolver{Resolver: reso}
+	}
+	return reso
+}
+
 // BogonResolver is a bogon aware resolver. When a bogon is encountered in
 // a reply, this resolver will return ErrDNSBogon.
 //

internal/netxlite/bogon_test.go

@@ -9,6 +9,25 @@ import (
 	"github.com/ooni/probe-cli/v3/internal/model/mocks"
 )
 
+func TestMaybeWrapWithBogonResolver(t *testing.T) {
+	t.Run("with enabled equal to true", func(t *testing.T) {
+		underlying := &mocks.Resolver{}
+		reso := MaybeWrapWithBogonResolver(true, underlying)
+		bogoreso := reso.(*BogonResolver)
+		if bogoreso.Resolver != underlying {
+			t.Fatal("did not wrap")
+		}
+	})
+
+	t.Run("with enabled equal to false", func(t *testing.T) {
+		underlying := &mocks.Resolver{}
+		reso := MaybeWrapWithBogonResolver(false, underlying)
+		if reso != underlying {
+			t.Fatal("expected unmodified resolver")
+		}
+	})
+}
+
 func TestBogonResolver(t *testing.T) {
 	t.Run("LookupHost", func(t *testing.T) {
 		t.Run("with failure", func(t *testing.T) {