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/experiment/dash/download_test.go

@@ -0,0 +1,104 @@
+package dash
+
+import (
+	"bytes"
+	"context"
+	"errors"
+	"io/ioutil"
+	"net/http"
+	"net/url"
+	"testing"
+)
+
+func TestDownloadNewHTTPRequestFailure(t *testing.T) {
+	expected := errors.New("mocked error")
+	_, err := download(context.Background(), downloadConfig{
+		deps: FakeDeps{newHTTPRequestErr: expected},
+	})
+	if !errors.Is(err, expected) {
+		t.Fatal("not the error we expected")
+	}
+}
+
+func TestDownloadHTTPClientDoFailure(t *testing.T) {
+	expected := errors.New("mocked error")
+	txp := FakeHTTPTransport{err: expected}
+	_, err := download(context.Background(), downloadConfig{
+		deps: FakeDeps{httpTransport: txp, newHTTPRequestResult: &http.Request{
+			Header: http.Header{},
+			URL:    &url.URL{},
+		}},
+	})
+	if !errors.Is(err, expected) {
+		t.Fatal("not the error we expected")
+	}
+}
+
+func TestDownloadInternalError(t *testing.T) {
+	txp := FakeHTTPTransport{resp: &http.Response{StatusCode: 500}}
+	_, err := download(context.Background(), downloadConfig{
+		deps: FakeDeps{httpTransport: txp, newHTTPRequestResult: &http.Request{
+			Header: http.Header{},
+			URL:    &url.URL{},
+		}},
+	})
+	if !errors.Is(err, errHTTPRequestFailed) {
+		t.Fatal("not the error we expected")
+	}
+}
+
+func TestDownloadReadAllFailure(t *testing.T) {
+	expected := errors.New("mocked error")
+	txp := FakeHTTPTransport{resp: &http.Response{
+		Body:       ioutil.NopCloser(bytes.NewReader(nil)),
+		StatusCode: 200,
+	}}
+	_, err := download(context.Background(), downloadConfig{
+		deps: FakeDeps{
+			httpTransport: txp,
+			newHTTPRequestResult: &http.Request{
+				Header: http.Header{},
+				URL:    &url.URL{},
+			},
+			readAllErr: expected,
+		},
+	})
+	if !errors.Is(err, expected) {
+		t.Fatal("not the error we expected")
+	}
+}
+
+func TestDownloadSuccess(t *testing.T) {
+	txp := FakeHTTPTransport{resp: &http.Response{
+		Body:       ioutil.NopCloser(bytes.NewReader(nil)),
+		StatusCode: 200,
+	}}
+	result, err := download(context.Background(), downloadConfig{
+		deps: FakeDeps{
+			httpTransport: txp,
+			newHTTPRequestResult: &http.Request{
+				Header: http.Header{},
+				URL:    &url.URL{},
+			},
+			readAllResult: []byte("[]"),
+		},
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if result.elapsed <= 0 {
+		t.Fatal("invalid elapsed")
+	}
+	if result.received <= 0 {
+		t.Fatal("invalid received")
+	}
+	if result.requestTicks <= 0 {
+		t.Fatal("invalid requestTicks")
+	}
+	if result.serverURL == "" {
+		t.Fatal("invalid serverURL")
+	}
+	if result.timestamp <= 0 {
+		t.Fatal("invalid timestamp")
+	}
+}