Improvements to rrule_test

[sethaxen]

Sorry if this is a duplicate of an existing issue, but I'd like to recommend 2 improvements to rrule_test.

1. relax the equality check of the outputs to use isapprox. Sometimes a slightly different algorithm is used to compute the output when doing AD and will cause a strict equality test to fail. e.g. the reverse-mode derivatives of eigvals(::Symmetric) uses eigen(::Symmetric), which produces approximately the same eigvals but not identical: https://github.com/FluxML/Zygote.jl/blob/0b3e32d5c0f8e5ef3b061a6f84ed5505f5a202a0/src/lib/array.jl#L590-L593
2. If the function returns a tuple of outputs or any of the arguments are tuples, iterate over them.

I had adapted rrule_test for some Zygote functions and implemented these changes: https://gist.github.com/sethaxen/fa67e541c4a2a5e773b475349ed87fb9/677f58af483e270fc5bdc1a318825e9fef1b0256#file-zygote_power_series_tests-jl-L4-L48

[oxinabox]

Yes,
Indeed, somewhere on the swift forum I was having exactly this discussion about accuracy requirement

[nickrobinson251]

https://forums.swift.org/t/agreement-of-valuewithdifferential-and-value/31869