Introduce and use specialized `//@ ignore-auxiliary` for test support files instead of using `//@ ignore-test`
### Summary
Add a semantically meaningful directive for ignoring test *auxiliary* files. This is for auxiliary files that *participate* in actual tests but should not be built by `compiletest` (i.e. these files are involved through `mod xxx;` or `include!()` or `#[path = "xxx"]`, etc.).
### Motivation
A specialized directive like `//@ ignore-auxiliary` makes it way easier to audit disabled tests via `//@ ignore-test`.
- These support files cannot use the canonical `auxiliary/` dir because they participate in module resolution or are included, or their relative paths can be important for test intention otherwise.
Follow-up to:
- #139705
- #139783
- #139740
See also discussions in:
- [#t-compiler > Directive name for non-test aux files?](512773817)
- [#t-compiler > Handling disabled `//@ ignore-test` tests](512005974)
- [#t-compiler/meetings > [steering] 2025-04-11 Dealing with disabled tests](511717981)
### Remarks on remaining unconditionally disabled tests under `tests/`
After this PR, against commit 79a272c640, only **14** remaining test files are disabled through `//@ ignore-test`:
<details>
<summary>Remaining `//@ ignore-test` files under `tests/`</summary>
```
tests/debuginfo/drop-locations.rs
4://@ ignore-test (broken, see #128971)
tests/rustdoc/macro-document-private-duplicate.rs
1://@ ignore-test (fails spuriously, see issue #89228)
tests/rustdoc/inline_cross/assoc-const-equality.rs
3://@ ignore-test (FIXME: #125092)
tests/ui/match/issue-27021.rs
7://@ ignore-test (#54987)
tests/ui/match/issue-26996.rs
7://@ ignore-test (#54987)
tests/ui/issues/issue-49298.rs
9://@ ignore-test (#54987)
tests/ui/issues/issue-59756.rs
2://@ ignore-test (rustfix needs multiple suggestions)
tests/ui/precondition-checks/write.rs
5://@ ignore-test (unimplemented)
tests/ui/precondition-checks/read.rs
5://@ ignore-test (unimplemented)
tests/ui/precondition-checks/write_bytes.rs
5://@ ignore-test (unimplemented)
tests/ui/explicit-tail-calls/drop-order.rs
2://@ ignore-test: tail calls are not implemented in rustc_codegen_ssa yet, so this causes 🧊
tests/ui/panics/panic-short-backtrace-windows-x86_64.rs
3://@ ignore-test (#92000)
tests/ui/json/json-bom-plus-crlf-multifile-aux.rs
3://@ ignore-test Not a test. Used by other tests
tests/ui/traits/next-solver/object-soundness-requires-generalization.rs
2://@ ignore-test (see #114196)
```
</details>
Of these, most are either **unimplemented**, or **spurious**, or **known-broken**. The outstanding one is `tests/ui/json/json-bom-plus-crlf-multifile-aux.rs` which I did not want to touch in *this* PR -- that aux file has load-bearing BOM and carriage returns and byte offset matters. I think those test files that require special encoding / BOM probably are better off as `run-make` tests. See #139968 for that aux file.
### Review advice
- Best reviewed commit-by-commit.
- The directive name diverged from the most voted `//@ auxiliary` because I think that's easy to confuse with `//@ aux-{crate,dir}`.
r? compiler
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