Test the number of outputs in frule and rrule are correct

[mzgubic]

Closes #167

[mzgubic]

I've stupidly merged this before the Integration test finished. Turns out that some ChainRules tests get broken by this, so I've reverted the changes and tagged another patch version of ChainRulesTestUtils.

Essentially what happens inside ChainRules is that there are some rrules for functions with optional arguments, e.g.

function rrule(
    ::typeof(pinv),
    x::AbstractVector{T},
    tol::Real = 0,
) where {T<:Union{Real,Complex}}
    y = pinv(x, tol)
    function pinv_pullback(Δy)
        ∂x = sum(abs2, parent(y)) .* vec(Δy') .- 2real(y * Δy') .* parent(y)
        return (NoTangent(), ∂x, ZeroTangent())
    end
    return y, pinv_pullback
end

Here, even if pinv is called with one argument x, ZeroTangent() is returned for tol. The other failure is similar.