Tests/LibWeb: Verify dialog return value is correct in dialog test

Author: https://github.com/tcl3
Commit: https://github.com/SerenityOS/serenity/commit/f5266e0096
Pull-request: https://github.com/SerenityOS/serenity/pull/23204
Reviewed-by: https://github.com/trflynn89 ✅

Tests/LibWeb/Text/expected/form-method-dialog.txt

@@ -1,2 +1,4 @@
   dialog.open before form submit: true
+dialog.returnValue before form submit should be empty: true
 dialog.open after form submit: false
+dialog.returnValue after form submit: PASS

Tests/LibWeb/Text/input/form-method-dialog.html

@@ -1,7 +1,7 @@
 <script src="./include.js"></script>
 <dialog id="test-dialog" open>
     <form method="dialog">
-        <button id="submit-button">Submit</button>
+        <button id="submit-button" value="PASS">Submit</button>
     </form>
 </dialog>
 <script>
@@ -10,8 +10,9 @@
         const submitter = document.getElementById("submit-button");
         const form = document.forms[0];
         println(`dialog.open before form submit: ${dialog.open}`);
-        form.submit();
+        println(`dialog.returnValue before form submit should be empty: ${dialog.returnValue === ""}`);
+        form.requestSubmit(submitter);
         println(`dialog.open after form submit: ${dialog.open}`);
-
+        println(`dialog.returnValue after form submit: ${dialog.returnValue}`);
     });
 </script>