Use consistent pattern for startup in FX system tests

We have a few different patterns in use for system tests that need to startup the FX runtime, and in most cases, show a stage.

We should consider modifying the tests to use as consistent a pattern as possible to avoid subtle difference that could affect test stability.




```
    @BeforeClass
    public static void initFX() throws InterruptedException {
        startupLatch = new CountDownLatch(1);
        // Start the Application
        new Thread(() -> Application.launch(TestApp.class, (String[])null)).start();
        Assert.assertTrue(startupLatch.await(15, TimeUnit.SECONDS));
    }
```

NOTE: double check that the Application being launched has this code it its start() method:
```
stage.setOnShown(l -> {
                Platform.runLater(() -> startupLatch.countDown());
            });
```