get post data after submitting User Profile Form for success message
Not possible – any input values added to your profile editing form (such as test in your case) will be included in the POST request to the users/saveUser controller action, but they won't actually be included in the redirect request back to the page (or to whatever URL is in your form's hidden redirect input).

As a workaround you can always check for the Flash notice that the UsersController sets after the user is successfully saved:

{% if craft.session.getFlash('notice') == 'User saved.'|t %}
    &lt;p&gt;Success message!&lt;/p&gt;
{% endif %}