plots: test created html

[pared]

We might not be aware what are our rendering limitations (#6894).
Even if we fix it (#6900) we need to manually test it in order to be sure the problem has been fixed.

It seems to me that we could limit such problems if we were able to somehow test our HTML.
In the cases mentioned above, if we were able to parse js code, execute it and get stderr output we could determine whether our visualization was successful.

Another thing to consider would be: how to check if the support for images works as intended?

This issue needs some research on that topic. Ideally, we would like to test produced HTML as any other tests.

Related: iterative/dvc#6944

[daavoo]

Closing in favor of #25