testing/slogtest: tests for slog handlers

testing/slogtest: tests for slog handlers

Add a package for testing that a slog.Handler implementation
satisfies that interface's documented requirements.

Code copied from x/exp/slog/slogtest.

Updates #56345.

Change-Id: I89e94d93bfbe58e3c524758f7ac3c3fba2a2ea96
Reviewed-on: https://go-review.googlesource.com/c/go/+/487895
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Jonathan Amsterdam <jba@google.com>
Reviewed-by: Alan Donovan <adonovan@google.com>