Migrate JUnit assertion to AssertJ core

[VanRoy]

Overview

The test code combines JUnit and AssertJ assertions.
We want to unify the test assertions and make them more readable by using only AssertJ assertions.

[jmaniquet]

I would be interested in this (this may have been obvious after my comment in my other PR).
If it is up for grab, I will be able to submit a PR by next week-end.

[scordio]

OpenRewrite has a dedicated recipe that could help with this migration.

[jmaniquet]

OpenRewrite has a dedicated recipe that could help with this migration.

I will look into that thank you