Host the avatar test as a test suite; drop visual_novel_core dependency

- The Live2D avatar integration test (driver, overlay, test model "ren")
  moves here from the separate live2d_test mod as test_suites/avatar. It
  is mounted as the virtual mod live2d__avatar only with
  --itest=live2d_avatar; normal launches never load it.
- The runtime library needs only the interface mod; the content mod
  dependency is gone.

manifest.json

@@ -5,6 +5,9 @@
     "author": "ct",
     "description": "Live2D Cubism C# runtime: model loading, rendering, physics, animation, effects",
     "category": "library, visual",
-    "depends": ["visual_novel_core>=1", "interface>=1"],
-    "depends_native": ["native/*.dll", "native/*.so"]
+    "depends": ["interface>=1"],
+    "depends_native": ["native/*.dll", "native/*.so"],
+  "test_suites": [
+    {"id": "avatar", "activate": ["--itest=live2d_avatar"]}
+  ]
 }