Respect `-Z` unstable options in `rustdoc --test`
This PR makes rustdoc respect `-Z` unstable options when collecting doctests (`rustdoc --test`).
In the process I also realized that `--error-format` wasn't respected as well, making UI annotations impossible to write so I fixed that as well.
Best reviewed commit by commit.
Fixes https://github.com/rust-lang/rust/issues/147276
Fixes https://github.com/rust-lang/rust/issues/143930
r? fmease