upgrade ScalaTest to match what scala-2.11 branch has

we might as well, since it will help reduce merge conflicts between
that branch and this one (headless).

cherry-picked from 2.11 branch:
(cherry picked from commit 0b5d54dfe1720356b661a1f7afba607a62e75991)
(cherry picked from commit 4391792cff94c9ed8a1ffe174357c6cd447a16b2)