I'm surprised that no-one has mentioned this already, but if you don't attach a license to your code, whether it be product or test code, then others have no rights over the code.
Licensing code is about giving rights to others that they would not normally have. Your test code is automatically covered by copyright (in most countries around the world) even if you don't explicitly attach a copyright notice. Being copyrighted, other people cannot use, distribute or derive from that code.
By attaching a license to your test code, you give people the right to use, distribute and/or modify that code in accordance with the license terms.
I would recommend that you distribute your test code with your product code, under the same license. This is simplest for everyone. It also means that if people want to submit patches to you, then they can run your unit tests before they sent you a patch, which should help weeding out patches which break things.