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/libooniffi/ooniffi_test.go

@@ -0,0 +1,139 @@
+package main
+
+import (
+	"testing"
+)
+
+func TestTaskStartNullPointer(t *testing.T) {
+	if ooniffi_task_start_(nil) != nil {
+		t.Fatal("expected nil result here")
+	}
+}
+
+func TestTaskStartInvalidJSON(t *testing.T) {
+	settings := cstring("{")
+	defer freestring(settings)
+	if ooniffi_task_start_(settings) != nil {
+		t.Fatal("expected nil result here")
+	}
+}
+
+func TestTaskStartIdxWrapping(t *testing.T) {
+	settings := cstring(`{
+		"assets_dir": "../testdata/oonimkall/assets",
+		"log_level": "DEBUG",
+		"name": "Example",
+		"options": {
+			"software_name": "oonimkall-test",
+			"software_version": "0.1.0"
+		},
+		"state_dir": "../testdata/oonimkall/state",
+		"temp_dir": "../testdata/oonimkall/tmp"
+	}`)
+	defer freestring(settings)
+	o := setmaxidx()
+	// do twice and see if it's idempotent
+	if task := ooniffi_task_start_(settings); task != nil {
+		t.Fatal("expected nil task here")
+	}
+	if task := ooniffi_task_start_(settings); task != nil {
+		t.Fatal("expected nil task here")
+	}
+	restoreidx(o)
+}
+
+func TestTaskWaitForNextEventNullPointer(t *testing.T) {
+	if ooniffi_task_wait_for_next_event(nil) != nil {
+		t.Fatal("expected nil result here")
+	}
+}
+
+func TestTaskIsDoneNullPointer(t *testing.T) {
+	if ooniffi_task_is_done(nil) == 0 {
+		t.Fatal("expected true-ish result here")
+	}
+}
+
+func TestTaskInterruptNullPointer(t *testing.T) {
+	ooniffi_task_interrupt(nil) // mainly: we don't crash :^)
+}
+
+func TestEventSerializationNullPointer(t *testing.T) {
+	if ooniffi_event_serialization_(nil) != nil {
+		t.Fatal("expected nil result here")
+	}
+}
+
+func TestEventDestroyNullPointer(t *testing.T) {
+	ooniffi_event_destroy(nil) // mainly: we don't crash
+}
+
+func TestTaskDestroyNullPointer(t *testing.T) {
+	ooniffi_task_destroy(nil) // mainly: we don't crash
+}
+
+func TestExampleNormalUsage(t *testing.T) {
+	settings := cstring(`{
+		"assets_dir": "../testdata/oonimkall/assets",
+		"log_level": "DEBUG",
+		"name": "Example",
+		"options": {
+			"software_name": "oonimkall-test",
+			"software_version": "0.1.0"
+		},
+		"state_dir": "../testdata/oonimkall/state",
+		"temp_dir": "../testdata/oonimkall/tmp"
+	}`)
+	defer freestring(settings)
+	task := ooniffi_task_start_(settings)
+	if task == nil {
+		t.Fatal("expected non-nil task here")
+	}
+	for ooniffi_task_is_done(task) == 0 {
+		event := ooniffi_task_wait_for_next_event(task)
+		t.Logf("%s", gostring(ooniffi_event_serialization_(event)))
+		ooniffi_event_destroy(event)
+	}
+	ooniffi_task_destroy(task)
+}
+
+func TestExampleInterruptAndDestroy(t *testing.T) {
+	settings := cstring(`{
+		"assets_dir": "../testdata/oonimkall/assets",
+		"log_level": "DEBUG",
+		"name": "Example",
+		"options": {
+			"software_name": "oonimkall-test",
+			"software_version": "0.1.0"
+		},
+		"state_dir": "../testdata/oonimkall/state",
+		"temp_dir": "../testdata/oonimkall/tmp"
+	}`)
+	defer freestring(settings)
+	task := ooniffi_task_start_(settings)
+	if task == nil {
+		t.Fatal("expected non-nil task here")
+	}
+	ooniffi_task_interrupt(task)
+	ooniffi_task_destroy(task)
+}
+
+func TestExampleDestroyImmediately(t *testing.T) {
+	settings := cstring(`{
+		"assets_dir": "../testdata/oonimkall/assets",
+		"log_level": "DEBUG",
+		"name": "Example",
+		"options": {
+			"software_name": "oonimkall-test",
+			"software_version": "0.1.0"
+		},
+		"state_dir": "../testdata/oonimkall/state",
+		"temp_dir": "../testdata/oonimkall/tmp"
+	}`)
+	defer freestring(settings)
+	task := ooniffi_task_start_(settings)
+	if task == nil {
+		t.Fatal("expected non-nil task here")
+	}
+	ooniffi_task_destroy(task)
+}