Add basic regression tests

[topepo]

A few implementations, specifically torch and libsvm/kernlab, don't have good control over random number usage. Also, we have seen differences in results across intel and apple silicon chips (but that seems to be getting better).

We have some places where we programmatically write out results in-line. If the results change, our encoded conclusions might no longer be valid.

We can take a few key objects and save their results once their usage is finalized. Then we can use testthat to verify that those results are the same (or within some tolerance). Since the project is almost structured like an R package, this means that we can use devtools::test() to check for consistency of results.