Add dependency cenkalti/backoff

vendor/github.com/cenkalti/backoff/backoff_test.go

@@ -0,0 +1,27 @@
+package backoff
+
+import (
+	"testing"
+	"time"
+)
+
+func TestNextBackOffMillis(t *testing.T) {
+	subtestNextBackOff(t, 0, new(ZeroBackOff))
+	subtestNextBackOff(t, Stop, new(StopBackOff))
+}
+
+func subtestNextBackOff(t *testing.T, expectedValue time.Duration, backOffPolicy BackOff) {
+	for i := 0; i < 10; i++ {
+		next := backOffPolicy.NextBackOff()
+		if next != expectedValue {
+			t.Errorf("got: %d expected: %d", next, expectedValue)
+		}
+	}
+}
+
+func TestConstantBackOff(t *testing.T) {
+	backoff := NewConstantBackOff(time.Second)
+	if backoff.NextBackOff() != time.Second {
+		t.Error("invalid interval")
+	}
+}