Make obtaining the Testable interface actually part of the interface.

filter/count/tq.go

Index: filter/count/tq.go
diff --git a/filter/count/tq.go b/filter/count/tq.go
index 65f0955b054e86be5a6027f843ba783f7579a4a4..379e69d3dc8956e190a35b4775d766dca8725f8c 100644
--- a/filter/count/tq.go
+++ b/filter/count/tq.go
@@ -42,6 +42,10 @@ func (t *tqCounter) Stats(queueNames []string, cb tq.RawStatsCB) error {
 	return t.c.Stats.up(t.tq.Stats(queueNames, cb))
 }
 
+func (t *tqCounter) Testable() tq.Testable {
+	return t.tq.Testable()
+}
+
 // FilterTQ installs a counter TaskQueue filter in the context.
 func FilterTQ(c context.Context) (context.Context, *TQCounter) {
 	state := &TQCounter{}