Migrate tests to AssertJ

[philwebb]

Commits d7320de and 02850f3 for #22894 have already started the process of migrating exception checking tests to AssertJ but there's a lot of benefit in migrating all remaining assertions as well. Tests will be easier to read if they all use the same assertion style, and a wholesale migration to JUnit 5 will be much easier.

[sbrannen]

JUnit Jupiter assertions have been migrated to AssertJ now as well (5a1217c).

[sbrannen]

TestNG assertions have been migrated to AssertJ now as well (4f4427f).