Add testable interface for datastore.

service/datastore/raw_interface.go

Index: service/datastore/raw_interface.go
diff --git a/service/datastore/raw_interface.go b/service/datastore/raw_interface.go
index fee4138f86c3812f8afdb8230fab8be0d6de1ee3..a3db860d9587acaff05075cdecc1d557eb664fc7 100644
--- a/service/datastore/raw_interface.go
+++ b/service/datastore/raw_interface.go
@@ -192,4 +192,8 @@ type RawInterface interface {
 	//   - none keys of the keys are 'special' (use a kind prefixed with '__')
 	//   - cb is not nil
 	DeleteMulti(keys []Key, cb DeleteMultiCB) error
+
+	// Testable returns the Testable interface for the implementation, or nil if
+	// there is none.
+	Testable() Testable
 }