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/miniooni/README.md

@@ -0,0 +1,8 @@
+# miniooni
+
+This directory contains the source code of a simple CLI client that we
+use for research as well as for running QA scripts. We designed this tool
+to have a CLI similar to MK and OONI Probe v2.x to ease running Jafar
+scripts that check whether these tools behave similarly.
+
+See also libminiooni.

internal/engine/cmd/miniooni/main.go

@@ -0,0 +1,21 @@
+// Command miniooni is a simple binary for research and QA purposes
+// with a CLI interface similar to MK and OONI Probe v2.x.
+//
+// See also libminiooni, which is where we implement this CLI.
+package main
+
+import (
+	"fmt"
+	"os"
+
+	"github.com/ooni/probe-cli/v3/internal/engine/libminiooni"
+)
+
+func main() {
+	defer func() {
+		if s := recover(); s != nil {
+			fmt.Fprintf(os.Stderr, "%s", s)
+		}
+	}()
+	libminiooni.Main()
+}