Add a simple testing framework

[JohnnyMorganz]

It would be useful if batteries included a testing framework. A simple one will do for start with.

We can then use it to test lute API and std / batteries code

Maybe https://github.com/itsfrank/frktest is a good example

[aatxe]

I like frktest and it's a good starting point. This could be pure Luau. If we want to do something particularly exotic, similar to jest, this might mean we want to integrate a bit on the runtime side as well.