test/chan: avoid wrap-around in memstats comparison

The select2.go test assumed that the memory allocated between
its two samplings of runtime.ReadMemStats is strictly
increasing.  To avoid failing the tests when this is not true,
a greater-than check is introduced before computing the
difference in allocated memory.

R=golang-dev, r, cshapiro
CC=golang-dev
https://golang.org/cl/13701046

test/chan/select2.go

@@ -47,7 +47,8 @@ func main() {
 	runtime.GC()
 	runtime.ReadMemStats(memstats)
 
-	if memstats.Alloc-alloc > 1.1e5 {
+	// Be careful to avoid wraparound.
+	if memstats.Alloc > alloc && memstats.Alloc-alloc > 1.1e5 {
 		println("BUG: too much memory for 100,000 selects:", memstats.Alloc-alloc)
 	}
 }