add facility to test AbstractRNG's

This should make it easier to test rngs which have the `AbstractRNG` interface. 
I'm really not good at choosing names, so I will change them on request.
Do you mean "should" ? Would you like I change it to: 
"a wrapped `AbstractRNG` obtained via the function `wrap(rng, T)` where `T` is the type of the variates produced by `rng` that one wants tested"