csl: Bumped to scalatest 2.2.6
csl: Bumped to scalatest 2.2.6

Problem

We want to start cross-building for scala 2.12.x, but there isn't
a published 2.2.4 on 2.12.x

Solution

Bump to scalatest 2.2.6, where there is a published version on
2.12.x.

Result

Crossbuilding goes more smoothly!

RB_ID=835850

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