Add testable interface for datastore.

filter/count/rds.go

Index: filter/count/rds.go
diff --git a/filter/count/rds.go b/filter/count/rds.go
index a0c5a45618b781fd1e3422f3b59a66fae14bd1d2..f3ce9f8736cfbc22ecc544e62ad6db1d5bf4b4b7 100644
--- a/filter/count/rds.go
+++ b/filter/count/rds.go
@@ -65,6 +65,10 @@ func (r *dsCounter) PutMulti(keys []ds.Key, vals []ds.PropertyMap, cb ds.PutMult
 	return r.c.PutMulti.up(r.ds.PutMulti(keys, vals, cb))
 }
 
+func (r *dsCounter) Testable() ds.Testable {
+	return r.ds.Testable()
+}
+
 // FilterRDS installs a counter datastore filter in the context.
 func FilterRDS(c context.Context) (context.Context, *DSCounter) {
 	state := &DSCounter{}