If you would like to keep the statistics for the test group, I would recommend leaving the test group running with only one member page. This will in turn retain the URL for the test group.
However, if you do not care too much for the statistics and have found what you were looking for, what you can do is:
1. Unapprove the Test Group
2. Unapprove the Landing Pages in the Test Group
3. Write down URL of the Test Group
4. Delete the Test Group
5. Clone the winning Landing Page
6. Change the URL of that cloned Landing Page to reflect the deleted Test Group URL
This will give you a landing page that carries the same URL fo the deleted test group.
Hope that helps