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/httphostheader/httphostheader_test.go

@@ -0,0 +1,107 @@
+package httphostheader
+
+import (
+	"context"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	"github.com/apex/log"
+	"github.com/ooni/probe-cli/v3/internal/engine/internal/mockable"
+	"github.com/ooni/probe-cli/v3/internal/engine/model"
+)
+
+const (
+	softwareName    = "ooniprobe-example"
+	softwareVersion = "0.0.1"
+)
+
+func TestNewExperimentMeasurer(t *testing.T) {
+	measurer := NewExperimentMeasurer(Config{})
+	if measurer.ExperimentName() != "http_host_header" {
+		t.Fatal("unexpected name")
+	}
+	if measurer.ExperimentVersion() != "0.3.0" {
+		t.Fatal("unexpected version")
+	}
+}
+
+func TestMeasurerMeasureNoMeasurementInput(t *testing.T) {
+	measurer := NewExperimentMeasurer(Config{
+		TestHelperURL: "http://www.google.com",
+	})
+	err := measurer.Run(
+		context.Background(),
+		newsession(),
+		new(model.Measurement),
+		model.NewPrinterCallbacks(log.Log),
+	)
+	if err == nil || err.Error() != "experiment requires input" {
+		t.Fatal("not the error we expected")
+	}
+}
+
+func TestMeasurerMeasureNoTestHelper(t *testing.T) {
+	measurer := NewExperimentMeasurer(Config{})
+	measurement := &model.Measurement{Input: "x.org"}
+	err := measurer.Run(
+		context.Background(),
+		newsession(),
+		measurement,
+		model.NewPrinterCallbacks(log.Log),
+	)
+	if err != nil {
+		t.Fatal(err)
+	}
+	sk, err := measurer.GetSummaryKeys(measurement)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if _, ok := sk.(SummaryKeys); !ok {
+		t.Fatal("invalid type for summary keys")
+	}
+}
+
+func TestRunnerHTTPSetHostHeader(t *testing.T) {
+	var host string
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		host = r.Host
+		w.WriteHeader(200)
+	}))
+	defer server.Close()
+	measurer := NewExperimentMeasurer(Config{
+		TestHelperURL: server.URL,
+	})
+	measurement := &model.Measurement{
+		Input: "x.org",
+	}
+	err := measurer.Run(
+		context.Background(),
+		newsession(),
+		measurement,
+		model.NewPrinterCallbacks(log.Log),
+	)
+	if host != "x.org" {
+		t.Fatal("not the host we expected")
+	}
+	if err != nil {
+		t.Fatal(err)
+	}
+}
+
+func newsession() model.ExperimentSession {
+	return &mockable.Session{MockableLogger: log.Log}
+}
+
+func TestSummaryKeysGeneric(t *testing.T) {
+	measurement := &model.Measurement{TestKeys: &TestKeys{}}
+	m := &Measurer{}
+	osk, err := m.GetSummaryKeys(measurement)
+	if err != nil {
+		t.Fatal(err)
+	}
+	sk := osk.(SummaryKeys)
+	if sk.IsAnomaly {
+		t.Fatal("invalid isAnomaly")
+	}
+}