Add return types to test class methods

[weierophinney]

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

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Originally posted by @tux-rampage at zendframework/zend-di#40

[tux-rampage]

This will become a requirement when upgrading to PHPUnit 9

[tux-rampage]

This is part of the CS. The auto-fixer of phpcs added these in #21