[release-branch.go1.9] runtime: deflake TestPeriodicGC
[release-branch.go1.9] 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/68118
Run-TryBot: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>
Reviewed-by: Chris Broadfoot <0c2851a4f8ab4aa5606e69c0757771dd13dbe686@golang.org>
Reviewed-by: Austin Clements <7ea35d812706d9213868749011af1ed4fa2f6aa0@google.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

nawawi/go,nawawi/go,golang/go,nawawi/go,nawawi/go,golang/go,nawawi/go,golang/go,golang/go,nawawi/go,nawawi/go,golang/go,nawawi/go,golang/go,golang/go,golang/go