runtime: don't overwrite global stop channel in tests

This is used by TestStopTheWorldDeadlock, and the new
TestReadMetricsSched test accidentally modifies this global variable
instead of (as intended) defining a local one.

Change-Id: I7aaece83f285d051ad8b56b7591c76613c39613a
Cq-Include-Trybots: luci.golang.try:gotip-linux-amd64-longtest
Reviewed-on: https://go-review.googlesource.com/c/go/+/696556
Reviewed-by: Michael Pratt <mpratt@google.com>
TryBot-Bypass: Michael Knyszek <mknyszek@google.com>

src/runtime/metrics_test.go

@@ -1774,7 +1774,7 @@ func TestReadMetricsSched(t *testing.T) {
 	t.Run("waiting", func(t *testing.T) {
 		// Force waiting count to be high.
 		const waitingCount = 1000
-		stop = make(chan bool)
+		stop := make(chan bool)
 		for i := 0; i < waitingCount; i++ {
 			go func() { <-stop }()
 		}