LPS-44476 SearchResultUtil: introduce tests to document current behavior

Hi @sergiogonzalez!

This PR is for the **Search** subsystem. It's exactly the same as [this one](https://github.com/brianchandotcom/liferay-portal/pull/19037) (originally [this one](https://github.com/sergiogonzalez/liferay-portal/pull/1469)) but now with an extra commit to drop completely the "alternate key" nomenclature, seeing that [it wasn't making any sense](https://github.com/brianchandotcom/liferay-portal/pull/19037#issuecomment-45163394). (As you had already [predicted](https://github.com/ealonso/liferay-portal/pull/194#issue-34892279).)

Instead, I studied the business code and emerged a nomenclature around the concept of an "attachment owner". For the cases in question, the search result is an attachment: a comment message, or a document file entry. What I had originally called an "alternate key" in the search result actually seems to be the class/pk for the top entity that owns the found attachment. Can you please ask @ealonso if this conclusion matches what the Search team knows about those two fields?

Thanks!