…oping through DOM iterators

This helps avoid needing to pass so many parameters around.

They were starting to get unwieldy.

This also changes the builder to allow using
`x.py test src/test/rustdoc-ui/intra-doc`; before, it would panic that
no paths were found.

This will hopefully help users figure out what was wrong and how
to fix it.

- Remove the difference between `parent_item` and `current_item`; these
  should never have been different.
- Remove `current_item` from `resolve` and `variant_field` so that
  `Self` is only substituted in one place at the very start.
- Resolve the current item as a `DefId`, not a `HirId`. This is what
  actually fixed the bug.

Hacks:
- `clean` uses `TypedefItem` when it _really_ should be
  `AssociatedTypeItem`. I tried fixing this without success and hacked
  around it instead (see comments)
- This stringifies DefIds, then resolves them a second time. This is
  really silly and rustdoc should just use DefIds throughout. Fixing
  this is a larger task than I want to take on right now.

…brary tests

Fix intra-doc links for `Self` on cross-crate items and primitives

- Remove the difference between `parent_item` and `current_item`; these
  should never have been different.
- Remove `current_item` from `resolve` and `variant_field` so that
  `Self` is only substituted in one place at the very start.
- Resolve the current item as a `DefId`, not a `HirId`. This is what
  actually fixed the bug.

Hacks:
- `clean` uses `TypedefItem` when it _really_ should be
  `AssociatedTypeItem`. I tried fixing this without success and hacked
  around it instead (see comments)
- This second-guesses the `to_string()` impl since it wants
  fully-qualified paths. Possibly there's a better way to do this.

…ard-ux, r=jyn514

Make keyboard interactions in the settings menu more pleasant

rust-lang#78868 improved the keyboard interactions with the settings page. This PR goes a bit further by allowing more than just "space" to toggle the checkboxes.

r? `@jyn514`

Change ui test that are run-pass and that do not test the compiler to library tests

Part of rust-lang#76268, these are some of the relevant ui tests I found that can be replaced by library tests.

Note: this PR just moves the tests, I have not checked for any overlap between these tests and existing library tests. The only test I changed is `env_home_dir`, where I added code to restore the old home dir after testing.

All moved tests:

| ui test | library test file | test |
| --- | --- | --- |
| `const\ascii_ctype.rs` | `core\tests\ascii.rs` | `ascii_ctype_const` |
| `const\const-str-ptr.rs` | `alloc\tests\str.rs` | `const_str_ptr` |
| `assert-eq-trailing-comma.rs` | `core\tests\macros.rs` | `assert_eq_trailing_comma` |
| `assert-escape.rs` | `core\tests\macros.rs` | `assert_escape` |
| `assert-ne-trailing-comma.rs` | `core\tests\macros.rs` | `assert_ne_trailing_comma` |
| `atomic-access-bool.rs` | `core\tests\atomic.rs` | `atomic_access_bool` |
| `atomic-alignment.rs` | `core\tests\atomic.rs` | `atomic_alignment` |
| `atomic-compare_exchange.rs` | `core\tests\atomic.rs` | `atomic_compare_exchange` |
| ~~`atomic-print.rs`~~ | ~~`std\tests\process.rs`~~ | ~~`atomic_print`~~ |
| `bool.rs` | `core\tests\bool.rs` | `test_bool` |
| `bool_not.rs` | `core\tests\bool.rs` | `test_bool_not` |
| `char_unicode.rs` | `core\tests\unicode.rs` | `version` |
| `cmp-default.rs` | `core\tests\cmp.rs` | `cmp_default` |
| `deref-mut-on-ref.rs` | `core\tests\ops.rs` | `deref_mut_on_ref` |
| `deref-on-ref.rs` | `core\tests\ops.rs` | `deref_on_ref` |
| `env-home-dir.rs` | `std\tests\env.rs` | `env_home_dir` |
| ~~`env-vars.rs`~~ | ~~`std\tests\env.rs`~~ | ~~`env_vars`~~ |
| `extend-for-unit.rs` | `core\tests\iter.rs` | `extend_for_unit` |
| `offset_from.rs` | `core\tests\ptr.rs` | `offset_from` |
| `option-ext.rs` | `core\tests\option.rs` | `option_ext` |
| `result-opt-conversions.rs` | `core\tests\result.rs` | `result_opt_conversions` |
| `sleep.rs` | `std\tests\thread.rs` | `sleep` |
| ~~`try-wait.rs`~~ | ~~`std\tests\process.rs`~~ | ~~`try_wait`~~ |
| `utf8.rs` | `alloc\tests\str.rs` | `utf8` |
| `utf8_chars.rs` | `alloc\tests\str.rs` | `utf8_chars` |
| `wrapping-int-api.rs` | `core\tests\num\wrapping.rs` | `wrapping_int_api` |

…-core, r=jyn514

Remove const_fn_feature_flags test

## Overview

Helps with rust-lang#76268

I found `const_fn_feature_flags` is targeting feature-gate and remove it.

r? `@matklad`

Move const ip in ui test to unit test

Helps with rust-lang#76268

r? `@matklad`

…imulacrum

Validate lint docs separately.

This addresses some concerns raised in rust-lang#76549 (comment) about errors with the lint docs being confusing and cumbersome. Errors from validating the lint documentation were being generated during `x.py doc` (and `x.py dist`), since extraction and validation are being done in a single step. This changes it so that extraction and validation are separated, so that `x.py doc` will not error if there is a validation problem, and tests are moved to `x.py test src/tools/lint-docs`.

This includes the following changes:

* Separate validation to `x.py test`.
* Added some more documentation on how to more easily modify and test the docs.
* Added more help to the error messages to hopefully provide more information on how to fix things.

The first commit just moves the code around, so you may consider looking at the other commits for a smaller diff.

Move intra-doc link tests into a subdirectory

They were starting to get unwieldy.

r? `@Manishearth`