Avoid discovery errors for inner classes not annotated with @Nested

[marcphilipp]

Overview

• Fix StackOverflowError
• Avoid discovery errors for inner classes not annotated with @Nested

Resolves #4661.

---

I hereby agree to the terms of the JUnit Contributor License Agreement.

---

Definition of Done

• There are no TODOs left in the code
• Method preconditions are checked and documented in the method's Javadoc
• Coding conventions (e.g. for logging) have been followed
• Change is covered by automated tests including corner cases, errors, and exception handling
• Public API has Javadoc and @API annotations
• Change is documented in the User Guide and Release Notes

[sormuras]

Test cases seem to match the use-cases seen in the wild, implementation looks good too.