Ensure chain notifier is started before accessed.

The use case comes from the RPC layer that is ready before the
chain notifier which is used in the sub server.

sweep/test_utils.go

@@ -200,6 +200,11 @@ func (m *MockNotifier) Start() error {
 	return nil
 }
 
+// Started checks if started
+func (m *MockNotifier) Started() bool {
+	return true
+}
+
 // Stop the notifier.
 func (m *MockNotifier) Stop() error {
 	return nil