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.

test_suites/avatar/scenes/live2d_avatar_test.tscn

@@ -0,0 +1,6 @@
+[gd_scene format=3]
+
+[ext_resource type="Script" path="src/Live2DAvatarTestDriver.cs" id="1_driver"]
+
+[node name="Live2DAvatarTestDriver" type="Node2D"]
+script = ExtResource("1_driver")