Removed outdated ui test suite README, give reasons for disabled tests

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

@@ -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/b.rs

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

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

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

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::read 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

@@ -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/traits/next-solver/object-soundness-requires-generalization.rs

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