test: Introduce hs_client_note_connection_attempt_succeeded().

This commit paves the way for the e2e circuit unittests.

Add a stub for the prop224 equivalent of rend_client_note_connection_attempt_ended().

That function was needed for tests, since the legacy function would get
called when we attach streams and our client-side tests would crash with
assert failures on rend_data.

This also introduces hs_client.[ch] to the codebase.