Add a reflection-based unit test for subclasses increasing method visibility

Summary: Ref T6822. We can't actually commit this until we fix all the violations mentioned in that task. Depends on D11231.

Test Plan:

• Used code to identify violations.
• Hit exception under arc unit --everything.

Reviewers: btrahan, epriestley

Reviewed By: epriestley

Subscribers: Korvin, joshuaspence, epriestley

Maniphest Tasks: T6822

Differential Revision: https://secure.phabricator.com/D11052