LPS-44581 PingbackMethodImpl: introduce tests to document current behavior

Hi @sergiogonzalez!

This is the 2nd in the series of small chunks for Blogs (following [this one, merged](https://github.com/brianchandotcom/liferay-portal/pull/18850)) where we introduce tests documenting current behavior of PingbackMethodsImpl, and refactor it in smaller pieces so the Message Boards logic can be extracted away to the Comments API.

Already has the contributions from @rotty3000 and @marcellustavares reviews.