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/example/example_test.go

@@ -0,0 +1,67 @@
+package example_test
+
+import (
+	"context"
+	"errors"
+	"testing"
+	"time"
+
+	"github.com/apex/log"
+	"github.com/ooni/probe-cli/v3/internal/engine/experiment/example"
+	"github.com/ooni/probe-cli/v3/internal/engine/internal/mockable"
+	"github.com/ooni/probe-cli/v3/internal/engine/model"
+)
+
+func TestSuccess(t *testing.T) {
+	m := example.NewExperimentMeasurer(example.Config{
+		SleepTime: int64(2 * time.Millisecond),
+	}, "example")
+	if m.ExperimentName() != "example" {
+		t.Fatal("invalid ExperimentName")
+	}
+	if m.ExperimentVersion() != "0.1.0" {
+		t.Fatal("invalid ExperimentVersion")
+	}
+	ctx := context.Background()
+	sess := &mockable.Session{MockableLogger: log.Log}
+	callbacks := model.NewPrinterCallbacks(sess.Logger())
+	measurement := new(model.Measurement)
+	err := m.Run(ctx, sess, measurement, callbacks)
+	if err != nil {
+		t.Fatal(err)
+	}
+	sk, err := m.GetSummaryKeys(measurement)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if _, ok := sk.(example.SummaryKeys); !ok {
+		t.Fatal("invalid type for summary keys")
+	}
+}
+
+func TestFailure(t *testing.T) {
+	m := example.NewExperimentMeasurer(example.Config{
+		SleepTime:   int64(2 * time.Millisecond),
+		ReturnError: true,
+	}, "example")
+	ctx := context.Background()
+	sess := &mockable.Session{MockableLogger: log.Log}
+	callbacks := model.NewPrinterCallbacks(sess.Logger())
+	err := m.Run(ctx, sess, new(model.Measurement), callbacks)
+	if !errors.Is(err, example.ErrFailure) {
+		t.Fatal("expected an error here")
+	}
+}
+
+func TestSummaryKeysGeneric(t *testing.T) {
+	measurement := &model.Measurement{TestKeys: &example.TestKeys{}}
+	m := &example.Measurer{}
+	osk, err := m.GetSummaryKeys(measurement)
+	if err != nil {
+		t.Fatal(err)
+	}
+	sk := osk.(example.SummaryKeys)
+	if sk.IsAnomaly {
+		t.Fatal("invalid isAnomaly")
+	}
+}