ong/log: integrate with slog tests

[komuw]

• Use testing/slogtest, see: https://github.com/golang/example/blob/master/slog-handler-guide/README.md#testing
• Also check if we need to use pointer to mutex, see: https://github.com/golang/go/commit/847d40d699832a1e054bc08c498548eff6a73ab6
• Also check if we can eliminate the annoying double logIDs. Check if https://github.com/golang/example/blob/master/slog-handler-guide/README.md can help.

[komuw]

I think that in order to conform to testing/slogtest, we need to change tact.

func New(w io.Writer, maxMsgs int)  slog.Logger {
    buf := &bytes.Buffer{}
    jh := slog.NewJSONHandler(
        // Do not pass in `w` here, pass in a buf like bytes.Buffer
        buf,
        opts,
    )
   hdlr := handler{Handler: jh, cBuf: cbuf, logID: id.New()}
   l := slog.New(hdlr)

  return l.With(logIDFieldName, id)

func (h handler) Handle(ctx context.Context, r slog.Record) error {
    err := h.Handler.Handle(ctx, r) // this will use json-logger to log into an in memory buff
    h.cBuf.store(json.bytes.Buffer) // whatever string/byte was logged by json.logger, store it in memory
    if r.Level >= slog.LevelError {
       for item in h.cbuf{
          _, _ = h.w.Write([]byte(item))
       }
    }
}