JUnit 4 to JUnit 5 conversion

[Ismael Juma <is...@juma.me.uk>]

Hi all,

A heads up that most modules have been converted from JUnit 4 to JUnit 5.
See KAFKA-7339[1] for more details. The following guides are useful if you
want to learn more about JUnit 5 or how a given JUnit 4 feature works in
JUnit 5:

* https://junit.org/junit5/docs/current/user-guide/#migrating-from-junit4
* https://junit.org/junit5/docs/current/user-guide/

For the most part, JUnit 5 is more flexible and takes better advantage of
Java 8 features. One exception is class parameterization (versus method
parameterization where JUnit 5 is better)[2].

Please be careful about merging PRs that are old and have not had a recent
Jenkins build. Forcing a new Jenkins build should uncover any problems
(i.e. test changes that have to be updated before the merge).

Thanks to all the contributors who helped convert the modules. Having the
ability to write better tests is a significant benefit.

Ismael

[1] https://issues.apache.org/jira/browse/KAFKA-7339
[2] https://github.com/junit-team/junit5/issues/878