I just worked this out so thought I would share here.
You must locate the subscriber that you wish to make a test recipient via the Subscribers > Search and edit option.
Search for them via their email address (they will of course have to be a Subscriber, you can add them manually if not)
Then click the pencil icon alongside their entry in the search results.
In their details page set the second to last dropdown “Test subscriber” to Yes.
When you test your emails it will be sent to any subscribers that have that option set to Yes.