Track stderr color in UI tests

[scrabsha]

Disclaimer: the whole idea comes from @estebank on twitter (link). I'm just summarizing it here.

Now that we can emit super-fancy colored suggestions (PR #86532), it would be a good idea to track the color of the messages we emit for some special cases in the ui tests.

A good way to track the color of each character would be to generate an HTML file per test. This would be both human-friendly (reviewers can open the resulting HTML file on their browser) and testing framework-friendly (as every character color is tracked, we can spot regressions).

As suggested in the original tweet, explicit coloring can be triggered with a // generate-html header command. We can also add a -Z flag so that users can use it outside of rustc tests.

As a new rust-lang/rust contributor, I'd love to work on that, but I need some ideas of how to track colors from experienced rustc developers first.

Edits:

• 2021-08-25: after some discussions with @ estabank on twitter, it seems that we don't want to track the colors of every ui test, but rather the ones for some special cases.

[scrabsha]

After some research in the codebase, i think this could be implemented by adding a new variant to Destination and WritableDst to represent the fact that we're writing html code. This variant would contain a type responsible for figuring out what to write when asked to set a new color.

[estebank]

With #121877 we can now better do this, but there seems to be some level of platform dependence on the output: #122029 (comment)