Clean up after successful test run

[kot-lex]

Wouldn't it be more useful if we will delete .new.png and .diff.png for the successful tests?
Current approach requires running approve command every time even if all screenshots match the reference ones.

[unindented]

if all screenshots match the reference ones

Why do you need to approve if they're the same?

[kot-lex]

Just to prevent them from being accidentally committed into repo.
Another solution is to add *.diff.png and *.new.png into .gitignore, but what is the use of those files in case tests were passed?

[unindented]

Ah, we do have those files in our .gitignore, that's probably why this was a non-issue for us.

I'm open to merging a PR that removes those files after successful run though.