runtime: deflake TestPeriodicGC
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/68151
Run-TryBot: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>
Reviewed-by: Austin Clements <7ea35d812706d9213868749011af1ed4fa2f6aa0@google.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

akutz/go,akutz/go,akutz/go,Cofyc/go,cloudflare/go,akutz/go,CAFxX/go,cloudflare/go,josharian/go,CAFxX/go,golang/go,akutz/go,josharian/go,golang/go,cloudflare/go,nawawi/go,nawawi/go,nawawi/go,golang/go,Cofyc/go,CAFxX/go,nawawi/go,cloudflare/go,CAFxX/go,cloudflare/go,josharian/go,codestation/go,codestation/go,cloudflare/go,polyverse-security/go,christopher-henderson/Go,cloudflare/go,akutz/go,polyverse-security/go,codestation/go,josharian/go,CAFxX/go,josharian/go,codestation/go,nawawi/go,nawawi/go,CAFxX/go,akutz/go,Cofyc/go,Cofyc/go,nawawi/go,akutz/go,polyverse-security/go,CAFxX/go,polyverse-security/go,cloudflare/go,josharian/go,polyverse-security/go,codestation/go,polyverse-security/go,josharian/go,golang/go,josharian/go,golang/go,polyverse-security/go,Cofyc/go,CAFxX/go,Cofyc/go,codestation/go,Cofyc/go,golang/go,Cofyc/go,codestation/go,nawawi/go,golang/go,golang/go,codestation/go,polyverse-security/go