Migrate to JUnit 5

[znerd]

Suggestion: Migrate the code from JUnit 4 to JUnit 5.

See:

• https://junit.org/junit5/
• https://junit.org/junit5/docs/current/user-guide/#migrating-from-junit4
• https://www.baeldung.com/junit-5-migration
• https://blog.jetbrains.com/idea/2020/08/migrating-from-junit-4-to-junit-5/
• https://www.arhohuttunen.com/junit-5-migration/