Make frule_test take a sample output tangent for testing purposes

[oxinabox]

trafficstars

rrule_test takes in sample sensitivities (tangents) for all inputs,even though rrule. doesn't use them (since it is reverse mode) which gives us something to test accumulation against.

ffule_test should do the same.

needful to be more consistent in how we handle testign that: see #59

[willtebbutt]

I think you're correct, although for frule_test you would want to accept a tangent for the output.