add to verify simple routing works as expected

tests/test_single_page_apps.py

@@ -0,0 +1,18 @@
+from nicegui import ui
+from nicegui.testing import Screen
+
+
+def test_routing_url(screen: Screen):
+    @ui.outlet('/', on_navigate=lambda _: '/main')
+    def layout():
+        ui.label('main layout')
+        yield
+
+    @layout.view('/main')
+    def main_content():
+        ui.label('main content')
+
+    screen.open('/')
+    screen.should_contain('main layout')
+    screen.should_contain('main content')
+    assert '/main' in screen.selenium.current_url