Integrates jMock with JUnit 5.

To write a mock object test in JUnit 5, declare a non-private field of type JUnit5Mockery that holds a JUnit5Mockery and annotate the field with `@RegisterExtension`, as shown below. The Mockery will be verified after the test has run and before the fixture is torn down.

    public class JUnit5TestThatDoesSatisfyExpectations {

        \@RegisterExtension
        JUnit5Mockery context = new JUnit5Mockery();

        \@Mock
        private Runnable runnable;

        \@Test
        public void doesSatisfyExpectations() {
            context.checking(new Expectations() {{
                oneOf (runnable).run();
            }});

            runnable.run();
        }
    }