LPS-44581 PingbackMethodImpl: tests to document current behavior

From @arboliveira:

Hi @sergiogonzalez!

This resend is the first of several [small PRs](https://github.com/brianchandotcom/liferay-portal/pull/18640#issuecomment-43695266) for the Blogs subsystem. It contains only the PingbackMethodImpl characterization test, to [get that going](https://github.com/brianchandotcom/liferay-portal/pull/18640#issuecomment-43695409). As soon as it gets on master, I'll follow with more chunks breaking up the original refactoring in logical parts.

@rotty3000 This is the XML-RPC method. Spring wiring comes after this and [the other one](https://github.com/sergiogonzalez/liferay-portal/pull/1447).