time: increase timeout in negative sleep duration test

There's enough jitter in the scheduler on overloaded machines
that 25ms is not enough.

LGTM=dave
R=golang-codereviews, gobot, rsc, dave
CC=golang-codereviews
https://golang.org/cl/83300044

src/pkg/time/sleep_test.go

@@ -347,19 +347,18 @@ func TestReset(t *testing.T) {
 // Test that sleeping for an interval so large it overflows does not
 // result in a short sleep duration.
 func TestOverflowSleep(t *testing.T) {
-	const timeout = 25 * Millisecond
 	const big = Duration(int64(1<<63 - 1))
 	select {
 	case <-After(big):
 		t.Fatalf("big timeout fired")
-	case <-After(timeout):
+	case <-After(25 * Millisecond):
 		// OK
 	}
 	const neg = Duration(-1 << 63)
 	select {
 	case <-After(neg):
 		// OK
-	case <-After(timeout):
+	case <-After(1 * Second):
 		t.Fatalf("negative timeout didn't fire")
 	}
 }