[meta] Add return typehinting to the test codebase

Problem/Motivation

Among our efforts to add strict typing to core, tests are good early targets, because the majority of test code is internal API.

Proposed resolution

Add return typehints to the test codebase, first where non-disruptive, then where possibly disruptive (base classes, traits, etc.).

Remaining tasks

1. 📌 Add void return typehints to all test methods Active
2. 🌱 Add return typehints to protected test helper methods Active
3. 📌 Add appropriate return typehints to all @dataProvider methods Active
4. Decide how to handle test fixtures, both within and outside of test class files. (Probably merits its own child meta.)
5. Children being added as various followups to the above.
6. Enable SlevomatCodingStandard.TypeHints.ReturnTypeHint phpcs code sniff on */tests/*

API changes

Return typehints on methods in test code.

Release notes snippet

Maybe?