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/saver_test.go

@@ -0,0 +1,80 @@
+package engine
+
+import (
+	"errors"
+	"testing"
+
+	"github.com/apex/log"
+	"github.com/google/go-cmp/cmp"
+	"github.com/ooni/probe-cli/v3/internal/engine/model"
+)
+
+func TestNewSaverDisabled(t *testing.T) {
+	saver, err := NewSaver(SaverConfig{
+		Enabled: false,
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if _, ok := saver.(fakeSaver); !ok {
+		t.Fatal("not the type of Saver we expected")
+	}
+	m := new(model.Measurement)
+	if err := saver.SaveMeasurement(m); err != nil {
+		t.Fatal(err)
+	}
+}
+
+func TestNewSaverWithEmptyFilePath(t *testing.T) {
+	saver, err := NewSaver(SaverConfig{
+		Enabled:  true,
+		FilePath: "",
+	})
+	if err == nil || err.Error() != "saver: passed an empty filepath" {
+		t.Fatalf("not the error we expected: %+v", err)
+	}
+	if saver != nil {
+		t.Fatal("saver should be nil here")
+	}
+}
+
+type FakeSaverExperiment struct {
+	M        *model.Measurement
+	Error    error
+	FilePath string
+}
+
+func (fse *FakeSaverExperiment) SaveMeasurement(m *model.Measurement, filepath string) error {
+	fse.M = m
+	fse.FilePath = filepath
+	return fse.Error
+}
+
+var _ SaverExperiment = &FakeSaverExperiment{}
+
+func TestNewSaverWithFailureWhenSaving(t *testing.T) {
+	expected := errors.New("mocked error")
+	fse := &FakeSaverExperiment{Error: expected}
+	saver, err := NewSaver(SaverConfig{
+		Enabled:    true,
+		FilePath:   "report.jsonl",
+		Experiment: fse,
+		Logger:     log.Log,
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if _, ok := saver.(realSaver); !ok {
+		t.Fatal("not the type of saver we expected")
+	}
+	m := &model.Measurement{Input: "www.kernel.org"}
+	if err := saver.SaveMeasurement(m); !errors.Is(err, expected) {
+		t.Fatalf("not the error we expected: %+v", err)
+	}
+	if diff := cmp.Diff(fse.M, m); diff != "" {
+		t.Fatal(diff)
+	}
+	if fse.FilePath != "report.jsonl" {
+		t.Fatal("passed invalid filepath")
+	}
+}