internal/testlog: fix log output from sub-loggers

[jwasinger]

When we instantiate a sub-logger via go-ethereum/internal/testlog/logger.With, we copy the reference to the bufHandler from the parent logger. However, internally, go-ethereum/internal/testlog/logger.With calls log/slog/Logger.With which creates a new handler instance (via internal/bufHandler.WithAttrs).

This PR modifies sub-logger instantiation to use the newly-instantiated handler, instead of copying the reference from the parent instance. The type cast from slog.Handler to *bufHandler in internal/testlog/Logger.With is safe here because a internal/testlog/Logger can only be instantiated with a *bufHandler as the underlying handler type.

Note, that I've also removed a pre-existing method that broke the above assumption. However, this method is not used in our codebase.

I'm not sure if the assumption holds for forks of geth (e.g. optimism has modified the testlogger somewhat allowing test loggers to accept arbitrary handler types), but it seems okay to break API compatibility given that this is in the internal package.

closes #31533

[jwasinger]

I also would like to modify the example unit test that I copied from the issue to be table-driven, but there doesn't appear to be a way to capture the testlog.T log output from a subtest in the parent test.