Add support for JUnit 5 Jupiter

[sbabcoc]

The lack of support for JUnit 5 is a serious deficiency at this point. Add this support, examining the possibility of migrating the existing support for TestNG and JUnit 4 into platform-specific subprojects.

[sbabcoc]

Create an extension, implementing these interfaces:

• Extension
• BeforeAllCallback
• BeforeEachCallback
• InvocationInterceptor
• AfterTestExecutionCallback
• AfterAllCallback
• TestWatcher

For an example, take a look at this.