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/internal/randx/randx_test.go

@@ -0,0 +1,34 @@
+package randx_test
+
+import (
+	"testing"
+
+	"github.com/ooni/probe-cli/v3/internal/engine/internal/randx"
+)
+
+func TestLetters(t *testing.T) {
+	str := randx.Letters(1024)
+	for _, chr := range str {
+		if (chr >= 'A' && chr <= 'Z') || (chr >= 'a' && chr <= 'z') {
+			continue
+		}
+		t.Fatal("invalid input char")
+	}
+}
+
+func TestLettersUppercase(t *testing.T) {
+	str := randx.LettersUppercase(1024)
+	for _, chr := range str {
+		if chr >= 'A' && chr <= 'Z' {
+			continue
+		}
+		t.Fatal("invalid input char")
+	}
+}
+
+func TestChangeCapitalization(t *testing.T) {
+	str := randx.Letters(2048)
+	if randx.ChangeCapitalization(str) == str {
+		t.Fatal("capitalization not changed")
+	}
+}