testing: b.Loop gives bogus results in some situations [1.24 backport]

[gopherbot]

@aclements requested issue #72971 to be considered for backport to the next 1.24 minor release.

@gopherbot, please open a backport to Go 1.24.

I'm on the fence about whether this should be backported. It makes b.Loop, which is new in Go 1.24, more robust to user mistakes, but those same mistakes could have been made with the old b.N style benchmarks, we just couldn't catch them. In support of backporting, it is a brand new feature and we're going to post a blog post about it soon so it'll probably get a spike in usage.

[aclements]

If we do want to backport this, we need to land backport #72934 first.

[gopherbot]

Change https://go.dev/cl/660556 mentions this issue: [release-branch.go1.24] testing: separate b.Loop counter from b.N

[gopherbot]

Change https://go.dev/cl/660557 mentions this issue: [release-branch.go1.24] testing: detect early return from B.Loop

[gopherbot]

Change https://go.dev/cl/660558 mentions this issue: [release-branch.go1.24] testing: detect a stopped timer in B.Loop

[gopherbot]

Closed by merging CL 660556 (commit 9204aca) to release-branch.go1.24.