Added js vtest on CI

[igorkorsukov]

No description provided.

[cbjeukendrup]

@igorkorsukov I'm afraid I see a problem:
So now you need to update the reference files when the tests are changed (that's annoying, but not the problem I mean).

But when the reference files are updated, the test on CI will pass without generating diff images, because there is no diff anymore then. So, once you have updated the reference files, you can't see anymore what has changed relative to the master branch. That's very inconvenient. Is there a solution for this?

The purpose of the VTests was namely not only automatically detecting changes, but also visually (not automatically) determining whether the changes are correct, by looking not only at the new situation, but also at the diff. It is important that we can do that.

(Also, it's regrettable that the "GIF" diffs are gone now, because those were actually the most useful in some cases. But it looks like those can be added back relatively easily.)