Update unit module scalatest version to 3.2.13

[rvacaru]

The scalatest version update, needed for the work done in #1650 , specifically because the old scalatest version in not found anymore.

When updating the versions there's a lib version conflict on scala-xml which is resolved by ignoring with a libraryDependencySchemes as suggested in the conflict output below:

sbt:scalafix> unit2_12Target3/update
[error] stack trace is suppressed; run last unit2_12Target3 / update for the full output
[error] (unit2_12Target3 / update) found version conflict(s) in library dependencies; some are suspected to be binary incompatible:
[error] 
[error]         * org.scala-lang.modules:scala-xml_2.12:2.1.0 (early-semver) is selected over {1.3.0, 1.0.6}
[error]             +- org.scalatest:scalatest-core_2.12:3.2.13           (depends on 2.1.0)
[error]             +- org.scala-lang:scala-compiler:2.12.16              (depends on 1.0.6)
[error]             +- io.get-coursier:coursier-core_2.12:2.0.0-RC5-6 (depends on xxx)
[error] this can be overridden using libraryDependencySchemes or evictionErrorLevel