test: use neorocks

[teto]

This allows to use a neovim-aware lua interpreter
https://github.com/nvim-neorocks/neorocks/

$ busted tests/ fails with

suite tests/main_spec.lua
...uajit-2.1.0-2022-10-04-env/share/lua/5.1/plenary/log.lua:9: attempt to index global 'vim' (a nil value)

stack traceback:
	...uajit-2.1.0-2022-10-04-env/share/lua/5.1/plenary/log.lua:9: in main chunk

but because it ignores the interpreter set in luarocks, which is why the command to run really should be luarocks test but in my case it tries to install the busted package and fails: the luarocks config should be updated to contain busted (via nixpkgs' generateLuarocksConfig for instance).