Rollup merge of rust-lang#139705 - jieyouxu:tests-precursor, r=compiler-errors

Removed outdated ui test suite README, give reasons for disabled tests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- `tests/ui/README.md` is very outdated, test suite docs should consistently live in rustc-dev-guide.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/`.

Author: ChrisDenton

tests/ui/README.md

@@ -1,2 +1,2 @@
 //! Not used by compiler, this is used by bootstrap cli self-test.
-//@ ignore-test
+//@ ignore-test (used by bootstrap)

tests/ui/bootstrap/self-test/a.rs

@@ -1,2 +1,2 @@
 //! Not used by compiler, used by bootstrap cli self-test.
-//@ ignore-test
+//@ ignore-test (used by bootstrap)

tests/ui/bootstrap/self-test/b.rs

@@ -1,4 +1,4 @@
-//@ ignore-test
+//@ ignore-test (auxiliary)
 
 // Companion to allow-in-other-module.rs
 

tests/ui/lint/unknown-lints/other.rs

@@ -2,7 +2,7 @@
 //@ compile-flags: -Copt-level=3 -Cdebug-assertions=no -Zub-checks=yes
 //@ error-pattern: unsafe precondition(s) violated: ptr::read requires
 //@ revisions: null misaligned
-//@ ignore-test
+//@ ignore-test (unimplemented)
 
 use std::ptr;
 

tests/ui/precondition-checks/read.rs

@@ -2,7 +2,7 @@
 //@ compile-flags: -Copt-level=3 -Cdebug-assertions=no -Zub-checks=yes
 //@ error-pattern: unsafe precondition(s) violated: ptr::write requires
 //@ revisions: null misaligned
-//@ ignore-test
+//@ ignore-test (unimplemented)
 
 use std::ptr;
 

tests/ui/precondition-checks/write.rs

@@ -2,7 +2,7 @@
 //@ compile-flags: -Copt-level=3 -Cdebug-assertions=no -Zub-checks=yes
 //@ error-pattern: unsafe precondition(s) violated: ptr::write requires
 //@ revisions: null misaligned
-//@ ignore-test
+//@ ignore-test (unimplemented)
 
 use std::ptr;
 

tests/ui/precondition-checks/write_bytes.rs

@@ -1,5 +1,5 @@
 //@ compile-flags: -Znext-solver
-//@ ignore-test
+//@ ignore-test (see #114196)
 
 trait Trait {
     type Gat<'lt>;