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/register_test.go

@@ -0,0 +1,63 @@
+package probeservices_test
+
+import (
+	"context"
+	"errors"
+	"strings"
+	"testing"
+
+	"github.com/ooni/probe-cli/v3/internal/engine/probeservices"
+	"github.com/ooni/probe-cli/v3/internal/engine/probeservices/testorchestra"
+)
+
+func TestMaybeRegister(t *testing.T) {
+	t.Run("when metadata is not valid", func(t *testing.T) {
+		clnt := newclient()
+		ctx := context.Background()
+		var metadata probeservices.Metadata
+		err := clnt.MaybeRegister(ctx, metadata)
+		if !errors.Is(err, probeservices.ErrInvalidMetadata) {
+			t.Fatal("expected an error here")
+		}
+	})
+	t.Run("when we have already registered", func(t *testing.T) {
+		clnt := newclient()
+		state := probeservices.State{
+			ClientID: "xx-xxx-x-xxxx",
+			Password: "xx",
+		}
+		if err := clnt.StateFile.Set(state); err != nil {
+			t.Fatal(err)
+		}
+		ctx := context.Background()
+		metadata := testorchestra.MetadataFixture()
+		if err := clnt.MaybeRegister(ctx, metadata); err != nil {
+			t.Fatal(err)
+		}
+	})
+	t.Run("when the API call fails", func(t *testing.T) {
+		clnt := newclient()
+		clnt.BaseURL = "\t\t\t" // makes it fail
+		ctx := context.Background()
+		metadata := testorchestra.MetadataFixture()
+		err := clnt.MaybeRegister(ctx, metadata)
+		if err == nil || !strings.HasSuffix(err.Error(), "invalid control character in URL") {
+			t.Fatal("expected an error here")
+		}
+	})
+}
+
+func TestMaybeRegisterIdempotent(t *testing.T) {
+	clnt := newclient()
+	ctx := context.Background()
+	metadata := testorchestra.MetadataFixture()
+	if err := clnt.MaybeRegister(ctx, metadata); err != nil {
+		t.Fatal(err)
+	}
+	if err := clnt.MaybeRegister(ctx, metadata); err != nil {
+		t.Fatal(err)
+	}
+	if clnt.RegisterCalls.Load() != 1 {
+		t.Fatal("called register API too many times")
+	}
+}