Ensure full AssertJ migrateability

[Philzen]

Although technically out of scope, many people would prefer their assertions to be assertJ-style in their project, either before or after applying the TestNG → Jupiter recipe.

The migration of the expectedExceptions and expectectedExceptionsMessageRegExp attributes as well as SkipExceptions would create JUnit-assertions/assumptions.

Possible approaches:

1. Implement a variant of the existing recipe
   • 👍 simple and straightforward
   • 👎 creates a second recipe list that would involve lots of duplicated code
2. Add input variables to the recipe (list), default being JUnit5-style, input option to use AssertJ
   • ❔ how would that be implemented
   • 👍 no code duplication
   • 👍 ideal performance
   • 👎 not straightforward for users, so we would wrap this into two recipe lists which comes down to 1.
3. Advise to run the TestNG → AssertJ assertions recipe after
   • ❕ need to ensure that recipe does pick up all replacements (this also involves checking some of the more specialized assertion migrations, such as Assert.assertEqualsNoOrder)
   • ❔ how does Add TestNG -> AssertJ rules openrewrite/rewrite-testing-frameworks#520 fit into this?
   • 👍 straightforward, keeps scope of this recipe