Switch to JUnit5

Since even Maven switched to JUnit5 [1], it'd be great to move forward and use JUnit5 in Enforcer too. The only problem is that it requires Java 8. [2]

Even if it won't happen with 3.x version, it's ok to put this task to backlog.

[1] https://github.com/apache/maven/commit/bb916d0784c7631866167928e4d0615df3317567

[2] https://junit.org/junit5/docs/current/user-guide/#overview-java-versions