Disclaimer: I am not a legal authority and the following is only my opinion based on my understanding of the developer license.

Each developer will need a licensed copy of the Developer Edition. Developer Edition can be used in the Test Environment, but it I remember correctly the testers do not need a licensed copy if they are not modifying, creating, or deleting database objects (tables, stored procedures, views, functions, and the like).

Your best option, however, is to check with Microsoft directly (even though you may get differing responses depending on who you yalk to and when).