The word from "an industry source" is that flight tests of the Sarmat missile will begin "in the beginning of 2019." This follows a series of ejection tests that took place in Plesetsk in December 2017, March 2018, and May 2018 (although the source quoted by TASS says there were only two tests).

UPDATE: The ministry of defense insists that it carried out only two Sarmat tests - in December 2017 and in March 2018.