I think things have gotten a little muddled in here. 
To create an admin in the manner the OP is suggesting is as simple as signing up a user in the regular fashion, activating their email manually from their user page (if you need to?), and granting admin powers at the same time. You can also temporarily disable reserved usernames if you want to use one of them when you sign them up. 
I create test users all the time, and it’s very quick and easy.
(and the plus address email trick @maiki suggested comes in useful too, or you can set up aliases if you have a domain inbox that supports that)