Exclude specific derivatives from tests #135
Description
Currently, it is not possible to use test_frule and test_rrule to check ChainRules definitions with missing derivatives that are implemented as @thunk(error("not implemented")) since the checks unthunk all derivatives automatically. This problem comes up e.g. in SpecialFunctions (JuliaMath/SpecialFunctions.jl#305).
It would be good if one could exclude specific derivatives from the checks completely (and not only from the finite differencing tests) and avoid that they are unthunked.