3rdparty: Actually upgrade scalatest to 3.1.2
3rdparty: Actually upgrade scalatest to 3.1.2

Problem

In the previous commit where I claimed to have upgraded to 3.1.2, I actually
upgraded to 3.1.1.  I also broke the sbt build :grimacing:

Solution

Let's actually move to 3.1.2.  We aren't moving to 3.2.0 yet because of the
breaking API changes related to suite names.

JIRA Issues: CSL-9722

Differential Revision: https://phabricator.twitter.biz/D518794

twitter/scrooge,twitter/scrooge,twitter/scrooge,twitter/scrooge