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/cmd/oohelperd/internal/fake_test.go

@@ -0,0 +1,127 @@
+package internal
+
+import (
+	"context"
+	"io"
+	"io/ioutil"
+	"net"
+	"net/http"
+	"time"
+
+	"github.com/ooni/probe-cli/v3/internal/engine/atomicx"
+	"github.com/ooni/probe-cli/v3/internal/engine/netx"
+)
+
+type FakeResolver struct {
+	NumFailures *atomicx.Int64
+	Err         error
+	Result      []string
+}
+
+func NewFakeResolverThatFails() FakeResolver {
+	return FakeResolver{NumFailures: atomicx.NewInt64(), Err: ErrNotFound}
+}
+
+func NewFakeResolverWithResult(r []string) FakeResolver {
+	return FakeResolver{NumFailures: atomicx.NewInt64(), Result: r}
+}
+
+var ErrNotFound = &net.DNSError{
+	Err: "no such host",
+}
+
+func (c FakeResolver) LookupHost(ctx context.Context, hostname string) ([]string, error) {
+	time.Sleep(10 * time.Microsecond)
+	if c.Err != nil {
+		if c.NumFailures != nil {
+			c.NumFailures.Add(1)
+		}
+		return nil, c.Err
+	}
+	return c.Result, nil
+}
+
+func (c FakeResolver) Network() string {
+	return "fake"
+}
+
+func (c FakeResolver) Address() string {
+	return ""
+}
+
+var _ netx.Resolver = FakeResolver{}
+
+type FakeTransport struct {
+	Err  error
+	Func func(*http.Request) (*http.Response, error)
+	Resp *http.Response
+}
+
+func (txp FakeTransport) RoundTrip(req *http.Request) (*http.Response, error) {
+	time.Sleep(10 * time.Microsecond)
+	if txp.Func != nil {
+		return txp.Func(req)
+	}
+	if req.Body != nil {
+		ioutil.ReadAll(req.Body)
+		req.Body.Close()
+	}
+	if txp.Err != nil {
+		return nil, txp.Err
+	}
+	txp.Resp.Request = req // non thread safe but it doesn't matter
+	return txp.Resp, nil
+}
+
+func (txp FakeTransport) CloseIdleConnections() {}
+
+var _ netx.HTTPRoundTripper = FakeTransport{}
+
+type FakeBody struct {
+	Data []byte
+	Err  error
+}
+
+func (fb *FakeBody) Read(p []byte) (int, error) {
+	time.Sleep(10 * time.Microsecond)
+	if fb.Err != nil {
+		return 0, fb.Err
+	}
+	if len(fb.Data) <= 0 {
+		return 0, io.EOF
+	}
+	n := copy(p, fb.Data)
+	fb.Data = fb.Data[n:]
+	return n, nil
+}
+
+func (fb *FakeBody) Close() error {
+	return nil
+}
+
+var _ io.ReadCloser = &FakeBody{}
+
+type FakeResponseWriter struct {
+	Body       [][]byte
+	HeaderMap  http.Header
+	StatusCode int
+}
+
+func NewFakeResponseWriter() *FakeResponseWriter {
+	return &FakeResponseWriter{HeaderMap: make(http.Header)}
+}
+
+func (frw *FakeResponseWriter) Header() http.Header {
+	return frw.HeaderMap
+}
+
+func (frw *FakeResponseWriter) Write(b []byte) (int, error) {
+	frw.Body = append(frw.Body, b)
+	return len(b), nil
+}
+
+func (frw *FakeResponseWriter) WriteHeader(statusCode int) {
+	frw.StatusCode = statusCode
+}
+
+var _ http.ResponseWriter = &FakeResponseWriter{}