Upgrade testing framework to JUnit Jupiter

The Commons BCEL project uses JUnit 4, with several tests still written in the style of JUnit 3 (test classes extending junit.framework.TestCase, etc.)

Converting the test suite to use the modern JUnit 5 Jupiter will make the tests easier to maintain and update. Additionally, as JUnit Jupiter is rapidly becoming the industry standard (if it isn't already), migrating the test suite to use it will make it easier to onboard new contributors.