test: deflake init1.go test, tighten its bounds

Per the comment at top, this test is about whether the GC runs during
init, but it was testing more than that, and testing how much the GC
collected in a certain amount of time.

Instead, loosen this test to just see whether it ran at all and not
how well it did.

Fixes #9848

Change-Id: I31da7dd769140d7b49aa6c149a543fae6076aa5e
Reviewed-on: https://go-review.googlesource.com/4820Reviewed-by: Rick Hudson <rlh@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

test/init1.go

@@ -17,22 +17,30 @@ func init() {
 	go send(c)
 	<-c
 
-	const chunk = 1 << 20
-	memstats := new(runtime.MemStats)
-	runtime.ReadMemStats(memstats)
-	sys := memstats.Sys
-	b := make([]byte, chunk)
+	const N = 1000
+	const MB = 1 << 20
+	b := make([]byte, MB)
 	for i := range b {
 		b[i] = byte(i%10 + '0')
 	}
 	s := string(b)
-	for i := 0; i < 1000; i++ {
+
+	memstats := new(runtime.MemStats)
+	runtime.ReadMemStats(memstats)
+	sys, numGC := memstats.Sys, memstats.NumGC
+
+	// Generate 1,000 MB of garbage, only retaining 1 MB total.
+	for i := 0; i < N; i++ {
 		x = []byte(s)
 	}
+
+	// Verify that the garbage collector ran by seeing if we
+	// allocated fewer than N*MB bytes from the system.
 	runtime.ReadMemStats(memstats)
-	sys1 := memstats.Sys
-	if sys1-sys > chunk*500 {
-		println("allocated 1000 chunks of", chunk, "and used ", sys1-sys, "memory")
+	sys1, numGC1 := memstats.Sys, memstats.NumGC
+	if sys1-sys >= N*MB || numGC1 == numGC {
+		println("allocated 1000 chunks of", MB, "and used ", sys1-sys, "memory")
+		println("numGC went", numGC, "to", numGC)
 		panic("init1")
 	}
 }