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/engine_suite_test.go

@@ -0,0 +1,17 @@
+package engine
+
+import (
+	"testing"
+
+	"github.com/deluan/navidrome/log"
+	"github.com/deluan/navidrome/tests"
+	. "github.com/onsi/ginkgo"
+	. "github.com/onsi/gomega"
+)
+
+func TestEngine(t *testing.T) {
+	tests.Init(t, false)
+	log.SetLevel(log.LevelCritical)
+	RegisterFailHandler(Fail)
+	RunSpecs(t, "Engine Suite")
+}