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/netx/resolver/tls_test.go

@@ -0,0 +1,32 @@
+package resolver
+
+import (
+	"context"
+	"crypto/tls"
+	"net"
+)
+
+func DialTLSContext(ctx context.Context, network, address string) (net.Conn, error) {
+	connch := make(chan net.Conn)
+	errch := make(chan error, 1)
+	go func() {
+		conn, err := tls.Dial(network, address, new(tls.Config))
+		if err != nil {
+			errch <- err
+			return
+		}
+		select {
+		case <-ctx.Done():
+			conn.Close()
+		case connch <- conn:
+		}
+	}()
+	select {
+	case <-ctx.Done():
+		return nil, ctx.Err()
+	case conn := <-connch:
+		return conn, nil
+	case err := <-errch:
+		return nil, err
+	}
+}