Upgrade to scalatest 3.2.0 for Scala 2.13.3 compatibility

We'll need to update to scalatest 3.2.0 in order to pick up the fix here, which fixes an incompatibility with Scala 2.13.3:

https://github.com/scalatest/scalatest/commit/7c89416aa9f3e7f2730a343ad6d3bdcff65809de

That's a big change unfortunately - 3.1 / 3.2 reorganized many classes. Fortunately it's just like import updates in 100 files.