Move engine package under subsonic, as it should only be used by the Subsonic API.master

The idea is to move reusable code from `engine` to `core`, in future refactorings

server/subsonic/engine/browser_test.go

@@ -0,0 +1,52 @@
+package engine
+
+import (
+	"context"
+	"errors"
+
+	"github.com/deluan/navidrome/model"
+	"github.com/deluan/navidrome/persistence"
+	. "github.com/onsi/ginkgo"
+	. "github.com/onsi/gomega"
+)
+
+var _ = Describe("Browser", func() {
+	var repo *mockGenreRepository
+	var b Browser
+
+	BeforeEach(func() {
+		repo = &mockGenreRepository{data: model.Genres{
+			{Name: "Rock", SongCount: 1000, AlbumCount: 100},
+			{Name: "", SongCount: 13, AlbumCount: 13},
+			{Name: "Electronic", SongCount: 4000, AlbumCount: 40},
+		}}
+		var ds = &persistence.MockDataStore{MockedGenre: repo}
+		b = &browser{ds: ds}
+	})
+
+	It("returns sorted data", func() {
+		Expect(b.GetGenres(context.TODO())).To(Equal(model.Genres{
+			{Name: "<Empty>", SongCount: 13, AlbumCount: 13},
+			{Name: "Electronic", SongCount: 4000, AlbumCount: 40},
+			{Name: "Rock", SongCount: 1000, AlbumCount: 100},
+		}))
+	})
+
+	It("bubbles up errors", func() {
+		repo.err = errors.New("generic error")
+		_, err := b.GetGenres(context.TODO())
+		Expect(err).ToNot(BeNil())
+	})
+})
+
+type mockGenreRepository struct {
+	data model.Genres
+	err  error
+}
+
+func (r *mockGenreRepository) GetAll() (model.Genres, error) {
+	if r.err != nil {
+		return nil, r.err
+	}
+	return r.data, nil
+}