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/collect_test.go

@@ -0,0 +1,113 @@
+package dash
+
+import (
+	"bytes"
+	"context"
+	"errors"
+	"io/ioutil"
+	"net/http"
+	"net/url"
+	"strings"
+	"testing"
+)
+
+func TestCollectJSONMarshalError(t *testing.T) {
+	expected := errors.New("mocked error")
+	deps := FakeDeps{jsonMarshalErr: expected}
+	err := collect(context.Background(), "", "", nil, deps)
+	if !errors.Is(err, expected) {
+		t.Fatal("not the error we expected")
+	}
+}
+
+func TestCollectNewHTTPRequestFailure(t *testing.T) {
+	expected := errors.New("mocked error")
+	deps := FakeDeps{newHTTPRequestErr: expected}
+	err := collect(context.Background(), "", "", nil, deps)
+	if !errors.Is(err, expected) {
+		t.Fatal("not the error we expected")
+	}
+}
+
+func TestCollectHTTPClientDoFailure(t *testing.T) {
+	expected := errors.New("mocked error")
+	txp := FakeHTTPTransport{err: expected}
+	deps := FakeDeps{httpTransport: txp, newHTTPRequestResult: &http.Request{
+		Header: http.Header{},
+		URL:    &url.URL{},
+	}}
+	err := collect(context.Background(), "", "", nil, deps)
+	if !errors.Is(err, expected) {
+		t.Fatal("not the error we expected")
+	}
+}
+
+func TestCollectInternalError(t *testing.T) {
+	txp := FakeHTTPTransport{resp: &http.Response{StatusCode: 500}}
+	deps := FakeDeps{httpTransport: txp, newHTTPRequestResult: &http.Request{
+		Header: http.Header{},
+		URL:    &url.URL{},
+	}}
+	err := collect(context.Background(), "", "", nil, deps)
+	if !errors.Is(err, errHTTPRequestFailed) {
+		t.Fatal("not the error we expected")
+	}
+}
+
+func TestCollectReadAllFailure(t *testing.T) {
+	expected := errors.New("mocked error")
+	txp := FakeHTTPTransport{resp: &http.Response{
+		Body:       ioutil.NopCloser(bytes.NewReader(nil)),
+		StatusCode: 200,
+	}}
+	deps := FakeDeps{
+		httpTransport: txp,
+		newHTTPRequestResult: &http.Request{
+			Header: http.Header{},
+			URL:    &url.URL{},
+		},
+		readAllErr: expected,
+	}
+	err := collect(context.Background(), "", "", nil, deps)
+	if !errors.Is(err, expected) {
+		t.Fatal("not the error we expected")
+	}
+}
+
+func TestCollectInvalidJSON(t *testing.T) {
+	txp := FakeHTTPTransport{resp: &http.Response{
+		Body:       ioutil.NopCloser(bytes.NewReader(nil)),
+		StatusCode: 200,
+	}}
+	deps := FakeDeps{
+		httpTransport: txp,
+		newHTTPRequestResult: &http.Request{
+			Header: http.Header{},
+			URL:    &url.URL{},
+		},
+		readAllResult: []byte("["),
+	}
+	err := collect(context.Background(), "", "", nil, deps)
+	if err == nil || !strings.HasSuffix(err.Error(), "unexpected end of JSON input") {
+		t.Fatal("not the error we expected")
+	}
+}
+
+func TestCollectSuccess(t *testing.T) {
+	txp := FakeHTTPTransport{resp: &http.Response{
+		Body:       ioutil.NopCloser(bytes.NewReader(nil)),
+		StatusCode: 200,
+	}}
+	deps := FakeDeps{
+		httpTransport: txp,
+		newHTTPRequestResult: &http.Request{
+			Header: http.Header{},
+			URL:    &url.URL{},
+		},
+		readAllResult: []byte("[]"),
+	}
+	err := collect(context.Background(), "", "", nil, deps)
+	if err != nil {
+		t.Fatal(err)
+	}
+}