Remove support for scalatest 3.0.x in testkit

[bjaglin]

From https://github.com/scalacenter/scalafix/pull/1650#discussion_r948473083

See https://github.com/scalacenter/scalafix/issues/1172#issuecomment-646621781 for the context of why we build/publish against an old version. For the next breaking change version of scalafix, I believe we should remove support for Scalafix 3.0.x (it's been nearly 3 years since the old Scalatest 3.0.x API is deprecated) by just building against the latest, but let's not change that in that PR.