Auto merge of #124847 - Oneirical:master, r=jieyouxu

Document tests in the `run-make` directory (A to C)

Part of the #121876 project.

This PR adds comments to some `run-make` tests which lack one, explaining _what_ is being tested. If possible, a link to the relevant PR or Issue responsible for the test is also provided.

This will help the porting efforts to `rmake.rs`, and will also allow maintainers to focus efforts on tests which are more pertinent to port. For example, [this test](https://github.com/rust-lang/rust/blob/master/tests/run-make/cat-and-grep-sanity-check/Makefile) will become useless after all tests containing `CGREP` are successfully ported.

In order to simplify review and at the suggestion of Kobzol on the rust-lang #gsoc Zulip, only the first 23 comments are part of this PR. If it is merged, future PRs will ensue commenting the rest of the tests.

Could be an UI test:

- `dep-info-doesnt-run-much`

Author: bors