[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 <rsc@golang.org>
Reviewed-by: Chris Broadfoot <cbro@golang.org>
Reviewed-by: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/runtime/gc_test.go

@@ -170,7 +170,7 @@ func TestPeriodicGC(t *testing.T) {
 	// slack if things are slow.
 	var numGCs uint32
 	const want = 2
-	for i := 0; i < 20 && numGCs < want; i++ {
+	for i := 0; i < 200 && numGCs < want; i++ {
 		time.Sleep(5 * time.Millisecond)
 
 		// Test that periodic GC actually happened.