test: enable standalone host discovery

Development~/UnityProject/Packages/manifest.json

@@ -11,5 +11,8 @@
   "dependencies": {
     "com.unity.test-framework": "1.1.33",
     "com.cneicy.shrink-shared-codegen": "file:../../.."
-  }
+  },
+  "testables": [
+    "com.cneicy.shrink-shared-codegen"
+  ]
 }