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")

test_suites/avatar/scenes/live2d_overlay.tscn

@@ -0,0 +1,13 @@
+[gd_scene format=3]
+
+[ext_resource type="Script" path="src/Live2DOverlay.cs" id="1_script"]
+
+[node name="Live2DOverlay" type="Control"]
+layout_mode = 3
+anchors_preset = 15
+anchor_right = 1.0
+anchor_bottom = 1.0
+grow_horizontal = 2
+grow_vertical = 2
+mouse_filter = 2
+script = ExtResource("1_script")