[release-branch.go1.8] runtime: deflake TestPeriodicGC

It was only waiting 0.1 seconds for the two GCs it wanted.
Let it wait 1 second.

Change-Id: Ib3cdc8127cbf95694a9f173643c02529a85063af
Reviewed-on: https://go-review.googlesource.com/68150


Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Chris Broadfoot <cbro@golang.org>
Reviewed-by: Austin Clements <austin@google.com>