(feat) JUnit: Add support classes, including an ApplicationTest base test class.

[hastebrot]

This PR adds some convenience classes for JUnit.

• It will be within the module testfx-junit.
• It will include an ApplicationTest class, that will serve as counterpart to JavaFX's Application. It launches the Toolkit per default and inherits methods from FxRobot.
• It will also include helpful classes for JUnit Categorys and Rules.

[hastebrot]

testfx-junit will compile with junit as dependency:

repositories {
    jcenter()
}

dependencies {
    testCompile "org.testfx:testfx-core:4.0.+"
    testCompile "org.testfx:testfx-junit:4.0.+"
}

ApplicationTest is the testing counter-part to Application:

public class HelloWorldDemo extends Application {
    @Override
    public void start(Stage stage) {
        Label label = new Label("Hello World!");
        StackPane root = new StackPane(label);
        Scene scene = new Scene(root, 400, 400);

        stage.setScene(scene);
        stage.show();
    }
}

public class HelloWorldDemoTest extends ApplicationTest {
    @Override
    public void start(Stage stage) {
        Label label = new Label("Hello World!");
        StackPane root = new StackPane(label);
        Scene scene = new Scene(root, 400, 400);

        stage.setScene(scene);
        stage.show();
    }

    @Test
    public void should_contain_label() {
        verifyThat(".label", hasText("Hello World!"));
    }
}

Use FxToolkit for special use-cases:

public class HelloWorldDemoTest extends ApplicationTest {
    @Override
    public void init() throws Exception {
        FxToolkit.registerStage(() -> new Stage());
    }

    @Override
    public void start(Stage stage) {
        Label label = new Label("Hello World!");
        StackPane root = new StackPane(label);
        Scene scene = new Scene(root, 400, 400);

        stage.setScene(scene);
        stage.show();
    }

    @Override
    public void stop() throws Exception {
        FxToolkit.hideStage();
    }

    @Test
    public void should_contain_label() {
        verifyThat(".label", hasText("Hello World!"));
    }
}

ApplicationTest.launch() is the testing counter-part to Application.launch():

public class HelloWorldMain {
    public static void main(String args) {
        Application.launch(HelloWorldDemo.class, args);
    }
}

public class HelloWorldMainTest {
    @Before
    public void setup() throws Exception {
        ApplicationTest.launch(HelloWorldDemo.class);
    }

    @After
    public void cleanup() throws Exception {
        FxToolkit.cleanupStages();
    }

    @Test
    public void should_contain_label() {
        verifyThat(".label", hasText("Hello World!"));
    }
}