Running nested JUnit tests

[John Hendrikx]

I would like to provide some junit tests in a PR.

These are written in a nested style for easy readability. For example:
    private StringProperty property = new SimpleStringProperty("A");

    @Nested
    class When_map_Called {

        @Nested
        class WithNull {

            @Test
            void shouldThrowNullPointerException() {
                assertThrows(NullPointerException.class, () -> 
property.map(null));
            }
        }

        @Nested
        class WithNotNullReturns_ObservableValue_Which {
            private ObservableValue<String> observableValue =
               property.map(v -> v + "Z");

            @Test
            void shouldNotBeNull() {
                assertNotNull(observableValue);
            }

            // etc...
        }

        // etc..
    }


Can this PR either:

1) Add JUnit 5 as a dependency

2) Use a custom JUnit 4 runner as a dependency that provides nested 
functionality

3) Add our own Nested runner based on an Apache 2.0 licensed one (about 
200 lines of test code).

For point 2 and 3, there is a good one here: 
https://github.com/NitorCreations/CoreComponents/tree/master/junit-runners)

Please let me know, I think tests are very important and having to tools 
to write them as readable as possible would be great.

--John