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/shellx/shellx.go

@@ -0,0 +1,66 @@
+// Package shellx contains utilities to run external commands.
+package shellx
+
+import (
+	"errors"
+	"os"
+	"os/exec"
+	"strings"
+
+	"github.com/apex/log"
+	"github.com/google/shlex"
+	"github.com/ooni/probe-cli/v3/internal/engine/model"
+)
+
+type runconfig struct {
+	args     []string
+	loginfof func(format string, v ...interface{})
+	name     string
+	stdout   *os.File
+	stderr   *os.File
+}
+
+func run(config runconfig) error {
+	config.loginfof("exec: %s %s", config.name, strings.Join(config.args, " "))
+	cmd := exec.Command(config.name, config.args...)
+	cmd.Stdout = config.stdout
+	cmd.Stderr = config.stderr
+	err := cmd.Run()
+	config.loginfof("exec result: %+v", err)
+	return err
+}
+
+// Run executes the specified command with the specified args
+func Run(name string, arg ...string) error {
+	return run(runconfig{
+		args:     arg,
+		loginfof: log.Log.Infof,
+		name:     name,
+		stdout:   os.Stdout,
+		stderr:   os.Stderr,
+	})
+}
+
+// RunQuiet is like Run but it does not emit any output.
+func RunQuiet(name string, arg ...string) error {
+	return run(runconfig{
+		args:     arg,
+		loginfof: model.DiscardLogger.Infof,
+		name:     name,
+		stdout:   nil,
+		stderr:   nil,
+	})
+}
+
+// RunCommandline is like Run but its only argument is a command
+// line that will be splitted using the google/shlex package
+func RunCommandline(cmdline string) error {
+	args, err := shlex.Split(cmdline)
+	if err != nil {
+		return err
+	}
+	if len(args) < 1 {
+		return errors.New("shellx: no command to execute")
+	}
+	return Run(args[0], args[1:]...)
+}

internal/engine/cmd/jafar/shellx/shellx_test.go

@@ -0,0 +1,44 @@
+package shellx
+
+import "testing"
+
+func TestRun(t *testing.T) {
+	if err := Run("whoami"); err != nil {
+		t.Fatal(err)
+	}
+	if err := Run("./nonexistent/command"); err == nil {
+		t.Fatal("expected an error here")
+	}
+}
+
+func TestRunQuiet(t *testing.T) {
+	if err := RunQuiet("true"); err != nil {
+		t.Fatal(err)
+	}
+	if err := RunQuiet("./nonexistent/command"); err == nil {
+		t.Fatal("expected an error here")
+	}
+}
+
+func TestRunCommandline(t *testing.T) {
+	t.Run("when the command does not parse", func(t *testing.T) {
+		if err := RunCommandline(`"foobar`); err == nil {
+			t.Fatal("expected an error here")
+		}
+	})
+	t.Run("when we have no arguments", func(t *testing.T) {
+		if err := RunCommandline(""); err == nil {
+			t.Fatal("expected an error here")
+		}
+	})
+	t.Run("when we have a single argument", func(t *testing.T) {
+		if err := RunCommandline("ls"); err != nil {
+			t.Fatal(err)
+		}
+	})
+	t.Run("when we have more than one argument", func(t *testing.T) {
+		if err := RunCommandline("ls ."); err != nil {
+			t.Fatal(err)
+		}
+	})
+}