testing/slogtest: add Run to run cases as subtests

testing/slogtest: add Run to run cases as subtests

This is an implementation of proposal #61758.

It adds a function to slogtest that runs each test case in a subtest,
instead of running them all at once.

That allows the caller to control which cases are run.

Fixes #61706.
Fixes #61758.

Change-Id: I95108b7b753675203ca7f0f00ccbc242bd9c2a9f
Reviewed-on: https://go-review.googlesource.com/c/go/+/516076
Reviewed-by: Alan Donovan <adonovan@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Jonathan Amsterdam <jba@google.com>