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/locale/en.properties

@@ -0,0 +1,2 @@
+mod_live2d__avatar_name = Live2D Test
+mod_live2d__avatar_desc = Test and demo mod for the live2d runtime: the sample Ren model (expressions, motions) shown by the home_outside overlay scene plus an avatar test driver.