There is page http://docs.grafana.org/features/datasources/testdata/
It’s said that in order to enable this feature, I need to go to /plugins/testdata/edit. Actually I need to go to Plugins/Apps/Grafana Test Data/ then open it and inside click enable.
Plugins/Apps/Grafana Test Data/
P.S. Maybe I am wrong.
This is fixed in 4.6 stable