Make `tezos-event-logging-test-helpers` a sublib of `tezos-event-logging`

The following discussion from !9311 (merged) should be addressed:

• @abate started a discussion: (+5 comments)

  above you used a dot in this case. so should this be tezos-event-logging.test-helpers ? I think either be consistent and use dot as directory separator, or just do not allow dot and use _ everywhere. I find the use of . and - to mean the same thing a bit confusing in the names of libs as they all get translated as _ in the end.