Make obtaining the Testable interface actually part of the interface.

impl/memory/taskqueue.go

Index: impl/memory/taskqueue.go
diff --git a/impl/memory/taskqueue.go b/impl/memory/taskqueue.go
index 2af0f5a7117f4345322c298b1d7ddad19003c98d..9ac3e5287d6b80ba66b140edee33f39d75a376ca 100644
--- a/impl/memory/taskqueue.go
+++ b/impl/memory/taskqueue.go
@@ -145,6 +145,10 @@ func (t *taskqueueImpl) Stats(queueNames []string, cb tq.RawStatsCB) error {
 	return nil
 }
 
+func (t *taskqueueImpl) Testable() tq.Testable {
+	return t
+}
+
 /////////////////////////////// taskqueueTxnImpl ///////////////////////////////
 
 type taskqueueTxnImpl struct {
@@ -222,6 +226,10 @@ func (t *taskqueueTxnImpl) Stats([]string, tq.RawStatsCB) error {
 	return errors.New("taskqueue: cannot Stats from a transaction")
 }
 
+func (t *taskqueueTxnImpl) Testable() tq.Testable {
+	return t
+}
+
 ////////////////////////////// private functions ///////////////////////////////
 
 var validTaskName = regexp.MustCompile("^[0-9a-zA-Z\\-\\_]{0,500}$")