Add modpack.txt as modpack support has been added to MineTest dev around an hour ago:

https://github.com/celeron55/minetest/commit/747ff7daf875652be1063fa49316125977b0ba79

This allows the user to simply copy the entire modpack folder into the mods/ directory, rather than copy every individual mod there instead.

modpack.txt

+The presence of this file indicates that the current folder is a modpack.
\ No newline at end of file