Add AssertJ for use in our own tests

[philwebb]

AssertJ provides a very nice fluent assertions style that in my opinion is easier to read and more consistent than Hamcrest. I'd like to add it as a test dependency and make use of it when testing the new MergedAnnotations API. Using AssertJ assertThatExceptionOfType... calls instead of @Rule ExpectedException will also help if we decide to migrate our tests to JUnit5.

Spring Boot tests have been written exclusively with AssertJ for some time now.

[sbrannen]

I'm totally in favor of using AssertJ in our own test suite. I miss it each time I write/update a test.

[sbrannen]

Team Decision: Allow the use of AssertJ within our test suite but do not migrate all existing assertions to AssertJ for the time being.