HelpWindowTest: fix failing test in non-headless mode

HelpWindowTest#focus_helpWindowNotFocused_focused() asserts that the
HelpWindow is not in focus when it is first shown, and then asserts
that the HelpWindow comes into focus after calling HelpWindow#focus().

When tests are run in non-headless mode, the HelpWindow will be in
focus when it is first shown, thus rendering our first assumption false
and causing the test to fail.

Let's update the test to focus on another TestFx Stage, which will
remove focus from the HelpWindow when it is first shown, ensuring that
the initial assumption is true.

src/test/java/seedu/address/ui/HelpWindowTest.java

@@ -46,8 +46,11 @@ public void isShowing_helpWindowIsHiding_returnsFalse() {
     }
 
     @Test
-    public void focus_helpWindowNotFocused_focused() {
+    public void focus_helpWindowNotFocused_focused() throws Exception {
         guiRobot.interact(helpWindow::show);
+
+        // Focus on another stage to remove focus from the helpWindow
+        FxToolkit.setupStage(Stage::requestFocus);
         assertFalse(helpWindow.getRoot().isFocused());
 
         guiRobot.interact(helpWindow::focus);