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/cmd/jafar/main_test.go

@@ -0,0 +1,89 @@
+package main
+
+import (
+	"errors"
+	"os"
+	"runtime"
+	"testing"
+
+	"github.com/ooni/probe-cli/v3/internal/engine/cmd/jafar/iptables"
+	"github.com/ooni/probe-cli/v3/internal/engine/cmd/jafar/shellx"
+)
+
+func ensureWeStartOverWithIPTables() {
+	iptables.NewCensoringPolicy().Waive()
+}
+
+func TestNoCommand(t *testing.T) {
+	if runtime.GOOS != "linux" {
+		t.Skip("skip test on non Linux systems")
+	}
+	ensureWeStartOverWithIPTables()
+	*dnsProxyAddress = "127.0.0.1:0"
+	*httpProxyAddress = "127.0.0.1:0"
+	*tlsProxyAddress = "127.0.0.1:0"
+	go func() {
+		mainCh <- os.Interrupt
+	}()
+	main()
+}
+
+func TestWithCommand(t *testing.T) {
+	if runtime.GOOS != "linux" {
+		t.Skip("skip test on non Linux systems")
+	}
+	ensureWeStartOverWithIPTables()
+	*dnsProxyAddress = "127.0.0.1:0"
+	*httpProxyAddress = "127.0.0.1:0"
+	*tlsProxyAddress = "127.0.0.1:0"
+	*mainCommand = "whoami"
+	defer func() {
+		*mainCommand = ""
+	}()
+	main()
+}
+
+func TestMustx(t *testing.T) {
+	t.Run("with no error", func(t *testing.T) {
+		var called int
+		mustx(nil, "", func(int) {
+			called++
+		})
+		if called != 0 {
+			t.Fatal("should not happen")
+		}
+	})
+	t.Run("with non-exit-code error", func(t *testing.T) {
+		var (
+			called   int
+			exitcode int
+		)
+		mustx(errors.New("antani"), "", func(ec int) {
+			called++
+			exitcode = ec
+		})
+		if called != 1 {
+			t.Fatal("not called?!")
+		}
+		if exitcode != 1 {
+			t.Fatal("unexpected exitcode value")
+		}
+	})
+	t.Run("with exit-code error", func(t *testing.T) {
+		var (
+			called   int
+			exitcode int
+		)
+		err := shellx.Run("curl", "-sf", "") // cause exitcode == 3
+		mustx(err, "", func(ec int) {
+			called++
+			exitcode = ec
+		})
+		if called != 1 {
+			t.Fatal("not called?!")
+		}
+		if exitcode != 3 {
+			t.Fatal("unexpected exitcode value")
+		}
+	})
+}