The ministry of defense told journalists that it conducted three missile tests at the Plesetsk test site since December 2017 - two ejection tests of Sarmat and one full test of RS-24 Yars.

This is the first official confirmation of the Yars test that apparently took place in June 2018. The test was not announced at the time, although it was confirmed informally.

The situation with Sarmat tests is more interesting - there was a report about a third ejection test in May 2018 and that information came from a number of normally credible sources and there is no reason to doubt them.

It's difficult to see why the ministry of defense would not admit that there was a test, especially since there is a video of a Sarmat ejection test that doesn't look like either December or March - the one that was published in the official ministry of defense YouTube channel on July 19, 2018:

We can definitely tell that this is not the December 2017 test - the video of that one was released on March 1, 2018, right after Putin's address:

There was also a video of the March test, published on March 29, 2018:

Even a quick look tells us that this is not the December test - the clouds are different to begin with and I'm sure that there are other differences too. We also know from satellite imagery that the site was covered in snow in March, so there is no way that first video was taken in March.