Merged test/ivs.lua into ltj-otf.lua

Merged test/ivs.lua into ltj-otf.lua

However, IVS-related code are not enabled if luatexja-otf.sty is only loaded.
To eneble it, one need to execute
  \directlua{luatexja.otf.enable_ivs()}
in the document.

I found that current IVS code does not work for OpenType fonts
  (e.g., Kozuka Mincho and  Hanazono Mincho OT).