tstime: add RandomDurationBetween helper

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

tstime/jitter_test.go

@@ -0,0 +1,23 @@
+// Copyright (c) 2021 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package tstime
+
+import (
+	"testing"
+	"time"
+)
+
+func TestRandomDurationBetween(t *testing.T) {
+	if got := RandomDurationBetween(1, 1); got != 1 {
+		t.Errorf("between 1 and 1 = %v; want 1", int64(got))
+	}
+	const min = 1 * time.Second
+	const max = 10 * time.Second
+	for i := 0; i < 500; i++ {
+		if got := RandomDurationBetween(min, max); got < min || got >= max {
+			t.Fatalf("%v (%d) out of range", got, got)
+		}
+	}
+}