test: add test to check that flow cleared (#19066)

Check that after navigation to
react view flow components are
not left on screen.

flow-tests/test-react-router/src/test/java/com/vaadin/flow/NavigationIT.java

@@ -254,6 +254,23 @@ public void testreactNavigationBrowserHistoryBack_anchor() {
                 $(SpanElement.class).first().getText());
     }
 
+    @Test
+    public void testReactNavigation_flowContentCleaned() {
+        open();
+
+        Assert.assertEquals("NavigationView",
+                $(SpanElement.class).first().getText());
+
+        $(AnchorElement.class).id(NavigationView.REACT_ANCHOR_ID).click();
+        Assert.assertEquals("This is a simple view for a React route",
+                $(ParagraphElement.class).id("react").getText());
+
+        Assert.assertFalse("Flow navigation view contents should not exist",
+                $(AnchorElement.class)
+                        .attribute("id", NavigationView.REACT_ANCHOR_ID)
+                        .exists());
+    }
+
     @Test
     public void testReactNavigationBrowserHistoryBack_serverNavigation() {
         open();