runtime: deflake TestStackMem

The problem is that there are lots of dead G's from previous tests,
each dead G consumes 1 stack segment.
Fixes #5034.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/7749043

src/pkg/runtime/stack_test.go

@@ -1576,7 +1576,9 @@ func TestStackMem(t *testing.T) {
 	if consumed > estimate {
 		t.Fatalf("Stack mem: want %v, got %v", estimate, consumed)
 	}
-	if s1.StackInuse > 4<<20 {
-		t.Fatalf("Stack inuse: want %v, got %v", 4<<20, s1.StackInuse)
+	inuse := s1.StackInuse - s0.StackInuse
+	t.Logf("Inuse %vMB for stack mem", inuse>>20)
+	if inuse > 4<<20 {
+		t.Fatalf("Stack inuse: want %v, got %v", 4<<20, inuse)
 	}
 }