Fix test log formatting.

`riak_test_lager_backend` was supporting Lager 2.0-style messages by manually extracting and then reconstituting the pieces. It's much easier just to pass it to `lager_default_formatter`, and then the format is more declarative. Since we also have the opportunity to inject newlines in each line as it is produced, we can skip the fold that
joined the log lines and simply convert it to a binary with the `unicode` module.

/cc @jaredmorrow @joedevivo