title: R: test on multiple versions
body: <p>Does anyone have a solution for automating unit testing of R packages under various multiple versions of R and/or prerequisite packages?</p>
  
  <p>Python has a nice package, <a href="https://tox.readthedocs.io/en/latest/" rel="nofollow noreferrer"><code>tox</code></a>, which allows the user to write a configuration file specifying what dimensions to expand over, then it runs testing under all combinations of those.</p>