sync: use blockUntilCleanupQueueEmpty instead of busy-looping in tests
testPool currently does the old-style busy loop to wait until cleanups
have executed. Clean this up by using the linkname'd
blockUntilCleanupQueueEmpty.
For #73642.
Change-Id: Ie0c2614db858a984f25b33a805dc52948069eb52
Reviewed-on: https://go-review.googlesource.com/c/go/+/671675 Reviewed-by: Michael Pratt <mpratt@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>