sync: give finalizers more time in TestPoolGC

sync: give finalizers more time in TestPoolGC

If we report a leak, make sure we've waited long enough to be sure.
The new sleep regimen waits 1.05 seconds before failing; the old
one waited 0.005 seconds.

(The single linux/amd64 failure in this test feels more like a
timing problem than a leak. I don't want to spend time on it unless
we're sure.)

LGTM=bradfitz
R=bradfitz
CC=golang-codereviews
https://golang.org/cl/72630043