Inconsistent use of validation libraries

[joelea]

We should go through and make use of a single validation library.

[nmiyake]

FWIW, I'm a big fan of https://github.com/google/truth -- very fluent/legible, broad compatibility, and find that the method-based completion for matchers makes it one of the easiest to write.

[dotCipher]

@joelea / @nmiyake I realize that the landscape has changed bit here, but fwiw isn't assertj more preferred now?

If so, we can relabel this issue and triage accordingly