chore: merge probe-engine into probe-cli (#201)

This is how I did it:

1. `git clone https://github.com/ooni/probe-engine internal/engine`

2. ```
(cd internal/engine && git describe --tags)
v0.23.0
```

3. `nvim go.mod` (merging `go.mod` with `internal/engine/go.mod`

4. `rm -rf internal/.git internal/engine/go.{mod,sum}`

5. `git add internal/engine`

6. `find . -type f -name \*.go -exec sed -i 's@/ooni/probe-engine@/ooni/probe-cli/v3/internal/engine@g' {} \;`

7. `go build ./...` (passes)

8. `go test -race ./...` (temporary failure on RiseupVPN)

9. `go mod tidy`

10. this commit message

Once this piece of work is done, we can build a new version of `ooniprobe` that
is using `internal/engine` directly. We need to do more work to ensure all the
other functionality in `probe-engine` (e.g. making mobile packages) are still WAI.

Part of https://github.com/ooni/probe/issues/1335

internal/engine/internal/mlablocate/mlablocate_test.go

@@ -0,0 +1,206 @@
+package mlablocate_test
+
+import (
+	"context"
+	"errors"
+	"io"
+	"net/http"
+	"strings"
+	"testing"
+
+	"github.com/apex/log"
+	"github.com/ooni/probe-cli/v3/internal/engine/internal/mlablocate"
+)
+
+func TestWithoutProxy(t *testing.T) {
+	client := mlablocate.NewClient(
+		http.DefaultClient,
+		log.Log,
+		"miniooni/0.1.0-dev",
+	)
+	result, err := client.Query(context.Background(), "ndt7")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if result.FQDN == "" {
+		t.Fatal("unexpected empty fqdn")
+	}
+}
+
+func Test404Response(t *testing.T) {
+	client := mlablocate.NewClient(
+		http.DefaultClient,
+		log.Log,
+		"miniooni/0.1.0-dev",
+	)
+	result, err := client.Query(context.Background(), "nonexistent")
+	if err == nil || !strings.Contains(err.Error(), "mlablocate: non-200 status code") {
+		t.Fatal("not the error we expected")
+	}
+	if result.FQDN != "" {
+		t.Fatal("expected empty fqdn")
+	}
+}
+
+func TestNewRequestFailure(t *testing.T) {
+	client := mlablocate.NewClient(
+		http.DefaultClient,
+		log.Log,
+		"miniooni/0.1.0-dev",
+	)
+	client.Hostname = "\t"
+	result, err := client.Query(context.Background(), "nonexistent")
+	if err == nil || !strings.Contains(err.Error(), "invalid URL escape") {
+		t.Fatal("not the error we expected")
+	}
+	if result.FQDN != "" {
+		t.Fatal("expected empty fqdn")
+	}
+}
+
+func TestHTTPClientDoFailure(t *testing.T) {
+	client := mlablocate.NewClient(
+		http.DefaultClient,
+		log.Log,
+		"miniooni/0.1.0-dev",
+	)
+	expected := errors.New("mocked error")
+	client.HTTPClient = &http.Client{
+		Transport: &roundTripFails{Error: expected},
+	}
+	result, err := client.Query(context.Background(), "nonexistent")
+	if !errors.Is(err, expected) {
+		t.Fatal("not the error we expected")
+	}
+	if result.FQDN != "" {
+		t.Fatal("expected empty fqdn")
+	}
+}
+
+type roundTripFails struct {
+	Error error
+}
+
+func (txp *roundTripFails) RoundTrip(*http.Request) (*http.Response, error) {
+	return nil, txp.Error
+}
+
+func TestCannotReadBody(t *testing.T) {
+	client := mlablocate.NewClient(
+		http.DefaultClient,
+		log.Log,
+		"miniooni/0.1.0-dev",
+	)
+	expected := errors.New("mocked error")
+	client.HTTPClient = &http.Client{
+		Transport: &readingBodyFails{Error: expected},
+	}
+	result, err := client.Query(context.Background(), "nonexistent")
+	if !errors.Is(err, expected) {
+		t.Fatal("not the error we expected")
+	}
+	if result.FQDN != "" {
+		t.Fatal("expected empty fqdn")
+	}
+}
+
+type readingBodyFails struct {
+	Error error
+}
+
+func (txp *readingBodyFails) RoundTrip(*http.Request) (*http.Response, error) {
+	return &http.Response{
+		StatusCode: 200,
+		Body:       &readingBodyFailsBody{Error: txp.Error},
+	}, nil
+}
+
+type readingBodyFailsBody struct {
+	Error error
+}
+
+func (b *readingBodyFailsBody) Read(p []byte) (int, error) {
+	return 0, b.Error
+}
+
+func (b *readingBodyFailsBody) Close() error {
+	return nil
+}
+
+func TestInvalidJSON(t *testing.T) {
+	client := mlablocate.NewClient(
+		http.DefaultClient,
+		log.Log,
+		"miniooni/0.1.0-dev",
+	)
+	client.HTTPClient = &http.Client{
+		Transport: &invalidJSON{},
+	}
+	result, err := client.Query(context.Background(), "nonexistent")
+	if err == nil || !strings.Contains(err.Error(), "unexpected end of JSON input") {
+		t.Fatal("not the error we expected")
+	}
+	if result.FQDN != "" {
+		t.Fatal("expected empty fqdn")
+	}
+}
+
+type invalidJSON struct{}
+
+func (txp *invalidJSON) RoundTrip(*http.Request) (*http.Response, error) {
+	return &http.Response{
+		StatusCode: 200,
+		Body:       &invalidJSONBody{},
+	}, nil
+}
+
+type invalidJSONBody struct{}
+
+func (b *invalidJSONBody) Read(p []byte) (int, error) {
+	if len(p) < 1 {
+		return 0, errors.New("slice too short")
+	}
+	p[0] = '{'
+	return 1, io.EOF
+}
+
+func (b *invalidJSONBody) Close() error {
+	return nil
+}
+
+func TestEmptyFQDN(t *testing.T) {
+	client := mlablocate.NewClient(
+		http.DefaultClient,
+		log.Log,
+		"miniooni/0.1.0-dev",
+	)
+	client.HTTPClient = &http.Client{
+		Transport: &emptyFQDN{},
+	}
+	result, err := client.Query(context.Background(), "nonexistent")
+	if err == nil || !strings.HasSuffix(err.Error(), "returned empty FQDN") {
+		t.Fatal("not the error we expected")
+	}
+	if result.FQDN != "" {
+		t.Fatal("expected empty fqdn")
+	}
+}
+
+type emptyFQDN struct{}
+
+func (txp *emptyFQDN) RoundTrip(*http.Request) (*http.Response, error) {
+	return &http.Response{
+		StatusCode: 200,
+		Body:       &emptyFQDNBody{},
+	}, nil
+}
+
+type emptyFQDNBody struct{}
+
+func (b *emptyFQDNBody) Read(p []byte) (int, error) {
+	return copy(p, []byte(`{"fqdn":""}`)), io.EOF
+}
+
+func (b *emptyFQDNBody) Close() error {
+	return nil
+}