Upgrade JUnit from version 4 to 5

Currently we are on JUnit 4 (version 4.12). JUnit 5 offers a bunch of benefits, including far better support for parameterized tests (can be per method instead of per class in JUnit 4), full Java 8 support, etc.

Migration would entail some work as some existing annotations have changed, support for '@Rule' has been dropped/replaced, etc. Of course none of this would affect code in production, just unit/integration tests.

Reference:

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