`RectPolynomial` tests for conversion/differentation don't test anything (other than it not erroring)

[DanielVandH]

https://github.com/JuliaApproximation/MultivariateOrthogonalPolynomials.jl/blob/ecd43a1bdac00f9fd98d15837aa4fbdd8054db27/test/test_rect.jl#L64C1-L90C8

I can probably add some at some point, but just making a note for now