Rollup merge of #135926 - jieyouxu:needs-subprocess-thread, r=oli-obk

Implement `needs-subprocess` directive, and cleanup a bunch of tests to use `needs-{subprocess,threads}`

### Summary

Closes #128295.

- Implements `//@ needs-subprocess` directive in compiletest as requested in #128295. However, compiletest is a host tool, so we can't just try to spawn process because that spawns the process on *host*, not the *target*, under cross-compilation scenarios.
    - The short-term solution is to add *Yet Another* list of allow-list targets.
    - The long-term solution is to first check if a `$target` supports std, then try to run a binary to do run-time capability detection *on the target*. But that is tricky because you have to build-and-run a binary *for the target*.
    - This PR picks the short-term solution, because the long-term solution is highly non-trivial, and it's already an improvement over individual `ignore-*`s all over the place.
    - Opened an issue about the long-term solution in #135928.
- Documents `//@ needs-subprocess` in rustc-dev-guide.
- Replace `ignore-{wasm,wasm32,emscripten,sgx}` with `needs-{subprocess,threads}` where suitable in tests.
- Some drive-by test changes as I was trying to figure out if I could use `needs-{subprocess,threads}` and found some bits needlessly distracting.

Count of tests that use `ignore-{wasm,wasm32,emscripten,sgx}` before and after this PR:

| State | `ignore-sgx` | `ignore-wasm` | `ignore-emscripten` |
| - | - | - | - |
| Before this PR | 96 | 88 | 207 |
| After this PR | 36 | 38 | 61 |

<details>
<summary>Commands used to find out locally</summary>

```
--- before

[17:40] Joe:rust (fresh) | rg --no-ignore -l "ignore-sgx" tests | wc -l
96
[17:40] Joe:rust (fresh) | rg --no-ignore -l "ignore-wasm" tests | wc -l
88
[17:40] Joe:rust (fresh) | rg --no-ignore -l "ignore-emscripten" tests | wc -l
207

--- after

[17:39] Joe:rust (needs-subprocess-thread) | rg --no-ignore -l "ignore-sgx" tests | wc -l
36
[17:39] Joe:rust (needs-subprocess-thread) | rg --no-ignore -l "ignore-wasm" tests | wc -l
38
[17:39] Joe:rust (needs-subprocess-thread) | rg --no-ignore -l "ignore-emscripten" tests | wc -l
61
```
</details>

### Review advice

- Best reviewed commit-by-commit.
- Non-trivial test changes (not mechanically simple replacements) are split into individual commits to help with review. Their individual commit messages give some basic description of the changes.
- I *could* split some test changes out into another PR, but I found that I needed to change some tests to `needs-threads`, some to `needs-subprocess`, and some needed to use *both*, so they might conflict and become very annoying.

---

r? ``@ghost`` (need to run try jobs)

try-job: x86_64-msvc-1
try-job: i686-msvc-1
try-job: i686-mingw
try-job: x86_64-mingw-1
try-job: x86_64-apple-1
try-job: aarch64-apple
try-job: aarch64-gnu
try-job: test-various
try-job: armhf-gnu

src/tests/directives.md

@@ -94,7 +94,7 @@ for more details.
 | Directive                         | Explanation                                                                                                              | Supported test suites                        | Possible values                                                                         |
 |-----------------------------------|--------------------------------------------------------------------------------------------------------------------------|----------------------------------------------|-----------------------------------------------------------------------------------------|
 | `check-run-results`               | Check run test binary `run-{pass,fail}` output snapshot                                                                  | `ui`, `crashes`, `incremental` if `run-pass` | N/A                                                                                     |
-| `error-pattern`                   | Check that output contains a specific string                                                                             | `ui`, `crashes`, `incremental` if `run-pass` | String                                                                                   |
+| `error-pattern`                   | Check that output contains a specific string                                                                             | `ui`, `crashes`, `incremental` if `run-pass` | String                                                                                  |
 | `regex-error-pattern`             | Check that output contains a regex pattern                                                                               | `ui`, `crashes`, `incremental` if `run-pass` | Regex                                                                                   |
 | `check-stdout`                    | Check `stdout` against `error-pattern`s from running test binary[^check_stdout]                                          | `ui`, `crashes`, `incremental`               | N/A                                                                                     |
 | `normalize-stderr-32bit`          | Normalize actual stderr (for 32-bit platforms) with a rule `"<raw>" -> "<normalized>"` before comparing against snapshot | `ui`, `incremental`                          | `"<RAW>" -> "<NORMALIZED>"`, `<RAW>`/`<NORMALIZED>` is regex capture and replace syntax |
@@ -176,6 +176,7 @@ settings:
 - `needs-rust-lld` — ignores if the rust lld support is not enabled (`rust.lld =
   true` in `config.toml`)
 - `needs-threads` — ignores if the target does not have threading support
+- `needs-subprocess`  — ignores if the target does not have subprocess support
 - `needs-symlink` — ignores if the target does not support symlinks. This can be
   the case on Windows if the developer did not enable privileged symlink
   permissions.