add notes about `rustdoc-ui` and `rustdoc-js` tests

[QuietMisdreavus]

While reviewing rust-lang/rust#50302 i noticed that these test folders aren't mentioned in the testing chapter at all. Since the linked PR adds a special configuration flag to the rustdoc-js tests, it would be worth mentioning it in the testing guide.

[jieyouxu]

Triage: rustdoc-js tests have now been documented by #2048. But AFAICT rustdoc-ui could still use some more elaboration.