op-service/testlog: pull in upstream geth features / improvements

The `op-service/testlog` package was forked off from the geth `testlog` package, since `testlog` is "internal", and cannot be relied on as dependency.

At the same time, the `testlog` in the monorepo has some opinionated improvements, like padding/alignment of filenames that are prefixed by the underlying test-logger.

Upstream, there are some buffer / locking related improvements, which we can merge back in.

Since the original fork, the `slog` package also became a thing in Go, and some features/flexibility of that may be nice to integrate also, if not already done in Geth.

Start by inspecting the op-service/testlog package and comparing it with upstream Geth's testlog implementation. Review the upstream buffer and locking changes, then assess whether relevant slog features are already present. Done means the selected compatible improvements are integrated without losing the package's existing formatting behavior.

Written by the indexing model from the issue text.