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/httptransport/logging_test.go

@@ -0,0 +1,77 @@
+package httptransport_test
+
+import (
+	"errors"
+	"io"
+	"io/ioutil"
+	"net/http"
+	"net/url"
+	"strings"
+	"testing"
+
+	"github.com/apex/log"
+	"github.com/ooni/probe-cli/v3/internal/engine/netx/httptransport"
+)
+
+func TestLoggingFailure(t *testing.T) {
+	txp := httptransport.LoggingTransport{
+		Logger: log.Log,
+		RoundTripper: httptransport.FakeTransport{
+			Err: io.EOF,
+		},
+	}
+	client := &http.Client{Transport: txp}
+	resp, err := client.Get("https://www.google.com")
+	if !errors.Is(err, io.EOF) {
+		t.Fatal("not the error we expected")
+	}
+	if resp != nil {
+		t.Fatal("expected nil response here")
+	}
+}
+
+func TestLoggingFailureWithNoHostHeader(t *testing.T) {
+	txp := httptransport.LoggingTransport{
+		Logger: log.Log,
+		RoundTripper: httptransport.FakeTransport{
+			Err: io.EOF,
+		},
+	}
+	req := &http.Request{
+		Header: http.Header{},
+		URL: &url.URL{
+			Scheme: "https",
+			Host:   "www.google.com",
+			Path:   "/",
+		},
+	}
+	resp, err := txp.RoundTrip(req)
+	if !errors.Is(err, io.EOF) {
+		t.Fatal("not the error we expected")
+	}
+	if resp != nil {
+		t.Fatal("expected nil response here")
+	}
+}
+
+func TestLoggingSuccess(t *testing.T) {
+	txp := httptransport.LoggingTransport{
+		Logger: log.Log,
+		RoundTripper: httptransport.FakeTransport{
+			Resp: &http.Response{
+				Body: ioutil.NopCloser(strings.NewReader("")),
+				Header: http.Header{
+					"Server": []string{"antani/0.1.0"},
+				},
+				StatusCode: 200,
+			},
+		},
+	}
+	client := &http.Client{Transport: txp}
+	resp, err := client.Get("https://www.google.com")
+	if err != nil {
+		t.Fatal(err)
+	}
+	ioutil.ReadAll(resp.Body)
+	resp.Body.Close()
+}