Rollup merge of #141597 - Oneirical:unquestionable-instruction, r=jieyouxu

Document subdirectories of UI tests with README files

Part of rust-lang/rust#133895 and the [2025 Google Summer of Code](https://blog.rust-lang.org/2025/05/08/gsoc-2025-selected-projects/) associated project.

When adding a new UI test, one is faced with hundreds of subdirectories in `tests/ui` reflecting various categories. Knowing where to put the new test is not trivial, as many of the categories have slightly misleading names. For example, `moves` does not only refer to the `move` keyword but to functions taking ownership in general, whereas `allocator` does not refer to allocation in general but rather to the very specific `allocator_api` and `global_allocator` features.

Many contributors will therefore place their test at the top level of ̀`tests/ui` where it will be mixed with hundreds of unrelated tests.

This PR is a tentative move towards more clearly defined tag/categories, with a SUMMARY.md file documenting the true purpose of each subdirectory, placed inside `tests/ui`.

r? ``@jieyouxu``

Author: matthiaskrgr

src/tests/ui.md

@@ -13,6 +13,11 @@ used for many other purposes. For example, tests can also be configured to [run
 the resulting program](#controlling-passfail-expectations) to verify its
 behavior.
 
+For a survey of each subdirectory's purpose under `tests/ui`, consult the
+[SUMMARY.md](https://github.com/rust-lang/rust/tree/master/tests/ui/SUMMARY.md).
+This is useful if you write a new test, and are looking for a category to
+place it in.
+
 If you need to work with `#![no_std]` cross-compiling tests, consult the
 [`minicore` test auxiliary](./minicore.md) chapter.