Removed outdated ui test suite README, give reasons for disabled tests
### Changes
- `tests/ui/README.md` is very outdated, test suite docs should consistently live in rustc-dev-guide.
- Add reasons for `//@ ignore-test` tests that don't have one.[^query]
This is a precursor change to make follow-up changes easier (possibly more specialized directives or converting some auxiliaries into the canonical `auxiliary/` form).
[^query]: searched via `rg --no-ignore -F -e "ignore-test" tests/`.
0e50fda4b6