This change adds the implementation for AddCleanup.Stop. It allows the
caller to cancel the call to execute the cleanup. Cleanup will not be
stopped if the cleanup has already been queued for execution.
For #67535
Change-Id: I494b77d344e54d772c41489d172286773c3814e5
Reviewed-on: https://go-review.googlesource.com/c/go/+/627975
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Michael Knyszek <mknyszek@google.com>
Auto-Submit: Carlos Amedee <carlos@golang.org>