CI: summarize by target language
Our [current CI](https://ci.kaitai.io/) grew to be too wide due to all pair combinations of ($TARGET, $VARIETY). Currently it's 33 pairs. It makes it hard to fit into a screen, it's not very easy to comprehend, given that we end up with a table of 33 cols * 220 rows = 7260 cells.

I propose to add a checkbox (enabled by default) to summarize these by ($TARGET). In case if results from different $VARIETY instances inside a $TARGET for a given test are different, we can highlight that as a severe error with a text like "mixed".