Unthunk cotangent components in findnz pullbacks

[Parvm1102]

The findnz pullbacks destructure the incoming tangent and pass the values component straight to sparse/sparsevec, but that component can itself be a thunk — unthunk on the outer tangent doesn't unthunk its components. For example:

using SparseArrays, Zygote
S = sprand(5, 5, 0.4)
Zygote.gradient(A -> sum(findnz(A)[3]), S)
# ERROR: MethodError: no method matching sparse(::Vector{Int64}, ::Vector{Int64}, ::InplaceableThunk{...}, ::Int64, ::Int64)

This unthunks the component in both the matrix and vector pullbacks and adds tests that call them with thunked components. With the fix, the gradient above returns the expected SparseMatrixCSC.