*rule_test should take a frule_like/rrule_like function so we can test AD systems

[oxinabox]

In order to make frule_test and rrule_test more generally useful for testing AD systems,
we should make them take a keyword argument that specifies what function that what to use.
This function would be used in the place of the default frule/rrule.

It would need to match the API of frule and rrule.
Just like the function that needs to be provided for JuliaDiff/ChainRulesCore.jl#68

Then we would be able to use these tools to test AD systems

[willtebbutt]

@oxinabox do you recall if this is included in ADIA? @matbesancon also requested it in #125 , and I keep realising that I need it when I want to check that AD is doing the right thing for a function that I particularly care about, even if I've not written any new rules.

[mzgubic]

I don't think it is, but we can probably add it - I have only considered this superficially but it doesn't seem like a huge task?

[willtebbutt]

Yeah, it really shouldn't be too large a task.

My opinion is that we should limit the scope such that we require whatever function is passed to have the same API as frule/ rrule, and its the responsibility of whoever is calling the tests to ensure that this is satisfied. I think that would already put us in a pretty good place.

If we do that, it ought to be a very quick job.