Fix UI test to be less brittle (#3299)

Author: Keboo

MaterialDesignThemes.UITests/WPF/ListBoxes/ListBoxTests.cs

@@ -168,7 +168,7 @@ public async Task OnToggle_ShouldGrabFocus()
         var listBoxItem = await listBox.GetElement<ListBoxItem>("/ListBoxItem[2]");
 
         await textBox.LeftClick();
-        Assert.True(await textBox.GetIsKeyboardFocusWithin());
+        await Wait.For(async () => Assert.True(await textBox.GetIsKeyboardFocusWithin()));
 
         // Act
         await listBoxItem.LeftClick();