test: Don't look for a specfic ANSI color

[Muscraft]

In rust-lang/rust#147207, I am changing rustc to anstyle for terminal styling. As part of this change, rustc will now be emitting 4-bit ANSI colors instead of 8-bit (256-color). That change is blocked on the message_format::cargo_renders_ansi test, which looks for a specific color sequence that rustc is no longer emitting and subsequently causes it to fail. To fix this, I made it so the test now looks for the start of an ANSI sequence instead of a specific sequence, as seeing the start of an ANSI escape sequence allows us to verify that ANSI colors are being emitted without the problems of looking for a specific sequence.

[rustbot]

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[weihanglo]

The build log btw: rust-lang/rust#147207 (comment)

2025-10-15T05:43:41.6679468Z ---- message_format::cargo_renders_ansi stdout ----
2025-10-15T05:43:41.6680580Z running `/checkout/obj/build/x86_64-unknown-linux-gnu/stage2-tools/x86_64-unknown-linux-gnu/release/cargo check --message-format json-diagnostic-rendered-ansi`
2025-10-15T05:43:41.6681526Z 
2025-10-15T05:43:41.6682062Z thread 'message_format::cargo_renders_ansi' (111799) panicked at src/tools/cargo/tests/testsuite/message_format.rs:124:10:
2025-10-15T05:43:41.6682742Z 
2025-10-15T05:43:41.6683644Z test failed running `/checkout/obj/build/x86_64-unknown-linux-gnu/stage2-tools/x86_64-unknown-linux-gnu/release/cargo check --message-format json-diagnostic-rendered-ansi`
2025-10-15T05:43:41.6693191Z error: expected to find:
2025-10-15T05:43:41.6693636Z [..]/u001b[38;5;9merror[..]
2025-10-15T05:43:41.6693884Z 
2025-10-15T05:43:41.6694034Z did not find in output:
2025-10-15T05:43:41.6700713Z {"reason":"compiler-message","package_id":"path+[ROOTURL]/foo#0.1.0","manifest_path":"[ROOT]/foo/Cargo.toml","target":{"kind":["bin"],"crate_types":["bin"],"name":"foo","src_path":"[ROOT]/foo/src/main.rs","edition":"2015","doc":true,"doctest":false,"test":true},"message":{"$message_type":"diagnostic","message":"`main` function not found in crate `foo`","code":{"code":"E0601","explanation":"No `main` function was found in a binary crate./n/nTo fix this error, add a `main` function:/n/n```/nfn main() {/n    // Your program will start here./n    println!(/"Hello world!/");/n}/n```/n/nIf you don't know the basics of Rust, you can look at the/n[Rust Book][rust-book] to get started./n/n[rust-book]: https://doc.rust-lang.org/book//n"},"level":"error","spans":[{"file_name":"src/main.rs","byte_start":0,"byte_end":0,"line_start":0,"line_end":0,"column_start":1,"column_end":1,"is_primary":true,"text":[],"label":null,"suggested_replacement":null,"suggestion_applicability":null,"expansion":null}],"children":[{"message":"consider adding a `main` function to `src/main.rs`","code":null,"level":"note","spans":[],"children":[],"rendered":null}],"rendered":"/u001b[1m/u001b[91merror[E0601]/u001b[0m/u001b[1m: `main` function not found in crate `foo`/u001b[0m/n  /u001b[1m/u001b[94m|/u001b[0m/n  /u001b[1m/u001b[94m= /u001b[0m/u001b[1mnote/u001b[0m: consider adding a `main` function to `src/main.rs`/n/n"}}
2025-10-15T05:43:41.6709831Z {"reason":"compiler-message","package_id":"path+[ROOTURL]/foo#0.1.0","manifest_path":"[ROOT]/foo/Cargo.toml","target":{"kind":["bin"],"crate_types":["bin"],"name":"foo","src_path":"[ROOT]/foo/src/main.rs","edition":"2015","doc":true,"doctest":false,"test":true},"message":{"$message_type":"diagnostic","message":"For more information about this error, try `rustc --explain E0601`.","code":null,"level":"failure-note","spans":[],"children":[],"rendered":"/u001b[1mFor more information about this error, try `rustc --explain E0601`./u001b[0m/n"}}
2025-10-15T05:43:41.6712336Z {"reason":"build-finished","success":false}