Add a tidy check to prevent adding new test files *directly* under `tests/ui/`

[jieyouxu]

Proposal

This MCP proposes to add a tidy check to prevent adding new test files directly under tests/ui/. Instead, we'll nudge contributors to either add them under existing subdirectories, or create more meaningful subdirectories to home them.

As part of rust-lang/rust#133895 and as of rust-lang/rust#143908, we no longer have test files immediately under tests/ui/, and instead only subdirectories. I would like to enforce this property (of no stray ui test files immediately under tests/ui) to make ui tests easier to find and easier to organize. In related efforts, we now also have a brief SUMMARY.md intended to briefly outline what each of the subdirectories under tests/ui/ are intended to home (rust-lang/rust#141597). This won't be perfect, but over time, I hope we could massage some of our ui test subdirectories to be more descriptive / organized.

Related discussion: #t-compiler > Discussion for ui test suite improvements

Mentors or Reviewers

I'm planning to either implement this check myself, or mentor/review it.

Process

The main points of the Major Change Process are as follows:

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• Once an MCP is seconded, the Final Comment Period begins.
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  • Outstanding concerns will block the Final Comment Period from finishing. Once all concerns are resolved, the 10 day countdown is restarted.
  • If no concerns are raised after 10 days since the resolution of the last outstanding concern, the MCP is considered approved.

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[rustbot]

Important

This issue is not meant to be used for technical discussion. There is a Zulip stream for that.
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<description of the concern>

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cc @rust-lang/compiler

[Urgau]

@rustbot second

[Kobzol]

@rustbot second

Yes please!