test: ensure all test methods have native return types

[paulbalandan]

Description
Enforce the phpdoc_to_return_type fixer to the tests. I have to manually fix some transformations particularly on unions with false changing to bool.

Checklist:

• Securely signed commits
• Component(s) with PHPDoc blocks, only if necessary or adds value
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide