runtime: fix TestLFStackStress

Fixes #7138.

R=r, bradfitz, dave
CC=dvyukov, golang-codereviews
https://golang.org/cl/53910043

src/pkg/runtime/lfstack_test.go

@@ -71,6 +71,8 @@ func TestLFStack(t *testing.T) {
 	}
 }
 
+var stress []*MyNode
+
 func TestLFStackStress(t *testing.T) {
 	const K = 100
 	P := 4 * GOMAXPROCS(-1)
@@ -80,14 +82,15 @@ func TestLFStackStress(t *testing.T) {
 	}
 	// Create 2 stacks.
 	stacks := [2]*uint64{new(uint64), new(uint64)}
-	// Need to keep additional referenfces to nodes, the stack is not all that type-safe.
-	var nodes []*MyNode
+	// Need to keep additional references to nodes,
+	// the lock-free stack is not type-safe.
+	stress = nil
 	// Push K elements randomly onto the stacks.
 	sum := 0
 	for i := 0; i < K; i++ {
 		sum += i
 		node := &MyNode{data: i}
-		nodes = append(nodes, node)
+		stress = append(stress, node)
 		LFStackPush(stacks[i%2], fromMyNode(node))
 	}
 	c := make(chan bool, P)
@@ -127,4 +130,7 @@ func TestLFStackStress(t *testing.T) {
 	if sum2 != sum {
 		t.Fatalf("Wrong sum %d/%d", sum2, sum)
 	}
+
+	// Let nodes be collected now.
+	stress = nil
 }