Make obtaining the Testable interface actually part of the interface.

filter/featureBreaker/tq.go

Index: filter/featureBreaker/tq.go
diff --git a/filter/featureBreaker/tq.go b/filter/featureBreaker/tq.go
index 6754d325f7e53c79de97b2e0c2ad683973997de1..d26f614926a556b00ea587baba5b826243f4c1af 100644
--- a/filter/featureBreaker/tq.go
+++ b/filter/featureBreaker/tq.go
@@ -34,6 +34,10 @@ func (t *tqState) Stats(queueNames []string, cb tq.RawStatsCB) error {
 	return t.run(func() error { return t.tq.Stats(queueNames, cb) })
 }
 
+func (t *tqState) Testable() tq.Testable {
+	return t.tq.Testable()
+}
+
 // FilterTQ installs a counter TaskQueue filter in the context.
 func FilterTQ(c context.Context, defaultError error) (context.Context, FeatureBreaker) {
 	state := newState(defaultError)