Update Fest Assert reference

[Capstan]

https://mvnrepository.com/artifact/org.easytesting/fest-assert/1.4 is very old and looks to be unmaintained. The http://fest.easytesting.org site now tries to sell you sesamin (a sesame-seed-based compound). There's no obvious source location, making it difficult to build from source inside Google's monorepo.

Figure out its successor and update tests accordingly.

One hint is from alexruiz/fest-assert-2.x#167 (comment) where one of the main contributors points would-be user to AssertJ.