compiletest: Do the known-directives check only once, and improve its error message

[Zalathar]

This PR is a combination of three changes:

• Store the list of known directives in a set, so that checking a directive name doesn't require a linear scan
• Extract the known-directives check out of iter_directives and do it only once, instead of multiple times per file
• Improve the error message for unknown directives, as requested on Zulip

The change to error messages is fused with the extraction, since doing it independently would have been more awkward.

---

Before

Testing stage1 with compiletest suite=coverage mode=coverage-map (aarch64-apple-darwin)
errors encountered during EarlyProps parsing: /Users/stuart/Dev/rust/rust/tests/coverage/trivial.rs
2025-11-05T01:55:46.440012Z ERROR compiletest::directives: /Users/stuart/Dev/rust/rust/tests/coverage/trivial.rs:2: detected unknown compiletest test directive `add-core-stubs`

thread '<unnamed>' (36268582) panicked at src/tools/compiletest/src/directives.rs:72:13:
errors encountered during EarlyProps parsing
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

After

Testing stage1 with compiletest suite=coverage mode=coverage-map (aarch64-apple-darwin)

thread '<unnamed>' (36270772) panicked at src/tools/compiletest/src/lib.rs:876:9:
directives check failed:
ERROR: unknown compiletest directive `add-core-stubs` at /Users/stuart/Dev/rust/rust/tests/coverage/trivial.rs:2
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

[rustbot]

Some changes occurred in src/tools/compiletest

cc @jieyouxu

compiletest directives have been modified. Please add or update docs for the
new or modified directive in src/doc/rustc-dev-guide/.

[rustbot]

r? @Kobzol

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[Zalathar]

This was originally going to wait until after #147955, but then Ralf and I both got annoyed by the error message, and improving the error message would be a lot harder without extracting the check first.