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/tcpconnect.go

@@ -0,0 +1,42 @@
+package internal
+
+import (
+	"context"
+	"sync"
+
+	"github.com/ooni/probe-cli/v3/internal/engine/experiment/webconnectivity"
+	"github.com/ooni/probe-cli/v3/internal/engine/netx"
+)
+
+// CtrlTCPResult is the result of the TCP check performed by the test helper.
+type CtrlTCPResult = webconnectivity.ControlTCPConnectResult
+
+// TCPResultPair contains the endpoint and the corresponding result.
+type TCPResultPair struct {
+	Endpoint string
+	Result   CtrlTCPResult
+}
+
+// TCPConfig configures the TCP connect check.
+type TCPConfig struct {
+	Dialer   netx.Dialer
+	Endpoint string
+	Out      chan TCPResultPair
+	Wg       *sync.WaitGroup
+}
+
+// TCPDo performs the TCP check.
+func TCPDo(ctx context.Context, config *TCPConfig) {
+	defer config.Wg.Done()
+	conn, err := config.Dialer.DialContext(ctx, "tcp", config.Endpoint)
+	if conn != nil {
+		conn.Close()
+	}
+	config.Out <- TCPResultPair{
+		Endpoint: config.Endpoint,
+		Result: CtrlTCPResult{
+			Failure: newfailure(err),
+			Status:  err == nil,
+		},
+	}
+}