Test arithmetic_closure against concrete types

[jishnub]

On Julia nightly,

julia> subtypes(AbstractFloat)
5-element Vector{Any}:
 BigFloat
 Core.BFloat16
 Float16
 Float32
 Float64

and Core.BFloat16 does not have one defined for itself, which was leading to test failures. It's better to be explicit in the types that we test against, which are known to work.

[mateuszbaran]

That's a good idea, though not having one for BFloat16 seems to be a bug in Julia? Or is there some justification for not having it?

[jishnub]

I'm not sure, but given that it's in Core and isn't public, it's probably some internal type. Perhaps it's best to not rely on its properties.