Explain the internal #[rustc_*] TEST attributes used for debugging and inside tests

[fmease]

First of all, I'm not entirely sure if it's worth documenting this at all.

I guess it could be useful for newcomers who haven't heard of them yet.
Well, I guess the non-exhaustive table I added can be of use for seasoned rustc devs, too.
In fact, I added the table for selfish reasons as I often found myself grepping through compiler/ and tests/ui/ in search of specific attributes whose names I keep forgetting (their naming scheme is slightly inconsistent).

These TEST attributes serve a dual purpose. Namely as a means to "printf+"-debug the compiler as well as in UI tests to perform "white-box testing" (sorta). Hence two sections on this topic. I chose compiler debugging as the "main" article here (with the testing one referring back to the other one) but I'm open to switch it around. I know that originally the TEST attrs were introduced for use in UI tests. However, I think the "compiler debugging" aspect is more important.

[jieyouxu]

Thank you!

I'm not entirely sure if it's worth documenting this at all.

I find these information very valuable, otherwise you'd either have to manage to stumble across this information on your own, or otherwise get passed down via review folklore (and has to be repeated every time someone is stuck on this). It's very hard to discover, realizing they even exist is a significant challenge in the first place. So thank you for taking the time and effort to document these!