So I hate the default adventurer skins, even if it's usually covered by armor and I use first-person camera. I have my own skin. But MineTest has no built-in config for skins! Mods to the rescue.
Back to menu, select each of those and Enable. Save.
Now in terminal:
cd ~/Library/Application Support/minetest/mods/skinsdb/textures cp ~/Pictures/My-Minecraft-Avatar.png character_whatever.png
This only seems to work with MC 1.0 avatars, even tho skinsdb says it works with MC 1.8; if you have multiple layers, remove those and resave without them. I tried installing the 3d_armor mod, which might enable that, and it has broken dependencies. 🙁
Play Game, and in game, open inventory, click the skin icon, and at the bottom of the big new Smart Inventory screen, skin icon. Select, done.