Testing Rules that use RuleConfigs

[oxinabox]

Need for testing JuliaDiff/ChainRules.jl#441 (comment)

We could use
#166
passing in rrule_f = (args...; kwargs...) -> rrule(ADviaRuleConfig(args...; kwargs...))

But probably we would like to have test_rrule's positional arguments continue matching rrule's.
and so we should make another PR that makes it take a optional first argument that is the config.
(and maybe by default it will be this ADviaRuleConfig ?)
If we do that #166 would want to be updated to also accept a config argument. which end user providing the rrule_f can ignore.
Or for testing their AD if they are also testing there config they can pass their config and set rrule_f=rrule_via_ad ?