Accept that NaN ≈ NaN in test_approx

[oxinabox]

I think this is the behavior we want.
If you get a actual value of NaN and you were expecting a NaN then all seems well.
If both the AD rule and the finite differencing take you here all is well.
(not ok if only one does)

[codecov-commenter]

Codecov Report

Merging #220 (447c9e9) into master (683e023) will decrease coverage by 0.33%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #220      +/-   ##
==========================================
- Coverage   90.75%   90.42%   -0.34%     
==========================================
  Files          11       11              
  Lines         303      303              
==========================================
- Hits          275      274       -1     
- Misses         28       29       +1     

Impacted Files	Coverage Δ
src/check_result.jl	88.23% <100.00%> (-1.48%)	⬇️

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[oxinabox]

This is not quite the same as JuliaDiff/FiniteDifferences.jl#192
which woulderror if the input is not finite.
Here we are allow output to be NaN,
so we could do both

[mzgubic]

needs a version bump and some tests?

[oxinabox]

idk if this is a good idea or not

[mzgubic]

The only thing I can think of going wrong is getting a NaN for the wrong reason. But seems relatively unlikely (and harmless in the end, at least at the tested point?)