Rehome tests in tests/ui/ to accurate subdirectories

[Oneirical]

Part of #133895 and the 2025 Google Summer of Code associated project.

tests/ui/ contains many top level tests without a "category", despite belonging in one of the subdirectories within. This pull request aims to reorganize top-level tests in accurate categories, according to the guidelines established by #141597.

Set to draft until it is decided whether it is better to release these PRs in one single large one, or multiple smaller ones for ease of reviewing.

The test double-ref.rs was also changed to be check-pass, as it does not execute any code at runtime.

r? @jieyouxu

[rustbot]

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[rust-log-analyzer]

The job mingw-check-tidy failed! Check out the build log: (web) (plain)

Click to see the possible cause of the failure (guessed by this bot)

[TIMING] core::build_steps::tool::Tidy { compiler: Compiler { stage: 0, host: x86_64-unknown-linux-gnu, forced_compiler: false }, target: x86_64-unknown-linux-gnu } -- 0.000
fmt check
fmt: checked 6077 files
tidy check
tidy error: Stray file with UI testing output: "/checkout/tests/ui/no_send-rc.stderr"
tidy error: Stray file with UI testing output: "/checkout/tests/ui/opt-in-copy.stderr"
tidy error: Stray file with UI testing output: "/checkout/tests/ui/paren-span.stderr"
tidy error: Stray file with UI testing output: "/checkout/tests/ui/tail-typeck.stderr"
tidy error: Stray file with UI testing output: "/checkout/tests/ui/maybe-bounds.stderr"
Running eslint on rustdoc JS files
tidy: Skipping binary file check, read-only filesystem
removing old virtual environment
creating virtual environment at '/checkout/obj/build/venv' using 'python3.10' and 'venv'
creating virtual environment at '/checkout/obj/build/venv' using 'python3.10' and 'virtualenv'
Requirement already satisfied: pip in ./build/venv/lib/python3.10/site-packages (25.1.1)
linting python files
All checks passed!
checking python file formatting
28 files already formatted
checking C++ file formatting
some tidy checks failed
Command has failed. Rerun with -v to see more details.
Build completed unsuccessfully in 0:01:34
  local time: Fri Jun 20 20:50:20 UTC 2025
  network time: Fri, 20 Jun 2025 20:50:21 GMT
##[error]Process completed with exit code 1.
Post job cleanup.

[fmease]

The illegal-sized-bounds directory is apparently meant for errors caused by trying to call methods on trait objects which are disabled via where Self: Sized — pretty niche (its entire content should be moved into the dyn-compatibility/ directory, probably).

I would move this test into trait-bounds/. Coincidentally I touched that file an hour ago in PR #142693, that's why I'm commenting on this at all.