feat: add option to fail on only

[DerYeger]

Clear and concise description of the problem

Sometimes an unwanted only used for writing tests may sneak into a commit.
If this isn't caught in the review process, potentially failing tests could be skipped by accident.

Suggested solution

Add an option --fail-on-only (or similar name) to fail a file if it contains an only test or suite.

I'd be happy to work on this if it's a desired feature.

Alternative

No response

Additional context

No response

Validations

• Follow our Code of Conduct
• Read the Contributing Guidelines.
• Read the docs.
• Check that there isn't already an issue that request the same feature to avoid creating a duplicate.