Fix return types and baselined errors of core/tests/ Unit code - round 6

Problem/Motivation

Adding return types to runtime code has BC concerns, but adding to test code should be straightforward and can be at least partially automated with rector.

My suggestion would be to do, for core/tests:

• #3551596: Add return types to core/tests code via Rector - round 1
• #3562361: Add type hints to core/tests code via Rector - round 2
• #3562868: Add types to class properties in core/tests code via Rector - round 3
• #3562645: Fix return types and baselined errors of core/tests/ Build|FunctionalJavascript|Functional code - round 4
• #3579189: Fix return types and baselined errors of core/tests/ Kernel code - round 5
• (this) - convert anything that is left out for Unit tests

Proposed resolution

Remove baseline entries and manually add missing return types and other fixes for Unit tests.

Note that some of the failures come from usage in tests of traits that live in the runtime namespaces, and cannot be fixed in this issue.

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet