Allow `test.warnings` and `test.assertions` to be defined as either a
list, or a function coerced to a list.
When defined as a function, it is supplied an `expect` function which
provides some syntactic-sugar for defining simple expectations.
This is an alternative to the current approach of defining that `expect`
function on an ad-hoc basis.
I prefer this to adding `expect` to nixvim's lib because:
1. That would require having access to `lib`
2. IDK where in `lib` such a specialized function should live