test_rrule doesn't catch returning wrong number of outputs

[oxinabox]

Consider a primal function with 2 inputs.
It's pullback should thus have 3 outputs (one for each input, plus one for the function itself).

test_rrule doesn't notice the following mistake

using ChainRulesTestUtils
using ChainRulesCore

foo(x, y) = x + 2y

function ChainRulesCore.rrule(::typeof(foo), x, y)
    foo_pullback(dz) = NoTangent(), dz

    # Should be
    #foo_pullback(dz) = NoTangent(), dz, 2dz

    return foo(x,y), foo_pullback
end

test_rrule(foo, 21.0, 32.0)

This is because zip in julia truncates to the shortest.
So as long as the ones that it does have are right it passes it.

The mistake is in:

ChainRulesTestUtils.jl/src/testers.jl

Line 207 in 1f50766

for (accumulated_x̄, x̄_ad, x̄_fd) in zip(accumulated_x̄, x̄s_ad, x̄s_fd)

We just need to add some extra tests before we get to that point