Add return types to test class methods

[tux-rampage]

It seems methods of Test-Classes should have return types, this is not the case within this project.
The TODO would be to add these return types.

Missing information

• Is there a commitment, rfc or discussion to refer to?
• Link to the originating PR

[weierophinney]

This repository has been closed and moved to laminas/laminas-di; a new issue has been opened at laminas/laminas-di#4.