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/probeservices/urls_test.go

@@ -0,0 +1,50 @@
+package probeservices_test
+
+import (
+	"context"
+	"strings"
+	"testing"
+
+	"github.com/ooni/probe-cli/v3/internal/engine/model"
+)
+
+func TestFetchURLListSuccess(t *testing.T) {
+	client := newclient()
+	client.BaseURL = "https://ams-pg-test.ooni.org"
+	config := model.URLListConfig{
+		Categories:  []string{"NEWS", "CULTR"},
+		CountryCode: "IT",
+		Limit:       17,
+	}
+	ctx := context.Background()
+	result, err := client.FetchURLList(ctx, config)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(result) != 17 {
+		t.Fatal("unexpected number of results")
+	}
+	for _, entry := range result {
+		if entry.CategoryCode != "NEWS" && entry.CategoryCode != "CULTR" {
+			t.Fatalf("unexpected category code: %+v", entry)
+		}
+	}
+}
+
+func TestFetchURLListFailure(t *testing.T) {
+	client := newclient()
+	client.BaseURL = "https://\t\t\t/" // cause test to fail
+	config := model.URLListConfig{
+		Categories:  []string{"NEWS", "CULTR"},
+		CountryCode: "IT",
+		Limit:       17,
+	}
+	ctx := context.Background()
+	result, err := client.FetchURLList(ctx, config)
+	if err == nil || !strings.HasSuffix(err.Error(), "invalid control character in URL") {
+		t.Fatal("not the error we expected")
+	}
+	if len(result) != 0 {
+		t.Fatal("results?!")
+	}
+}