device: run fewer trials in TestWaitPool when race detector enabled

On a many-core machine with the race detector enabled,
this test can take several minutes to complete.

Signed-off-by: Josh Bleecher Snyder <josh@tailscale.com>

device/race_enabled_test.go

@@ -0,0 +1,10 @@
+//+build race
+
+/* SPDX-License-Identifier: MIT
+ *
+ * Copyright (C) 2017-2021 WireGuard LLC. All Rights Reserved.
+ */
+
+package device
+
+const raceEnabled = true