Support individual files for clippy tests

[jyn514]

Clippy is a special snowflake and uses TESTNAME to tell it how to run individual tests instead of an argument to compiletest. Currently the only way to run individual tests is with TESTNAME=ice-6250 x test src/tools/clippy --bless. We should support x test src/tools/clippy/tests/ice-6250.rs as well.

#103266 will help simplify this a little by supporting the same test arguments as in-tree compiletest, but it won't solve suite paths automatically. Suite code lives around https://github.com/rust-lang/rust/blob/51de2387dc8b0871a7bbb58f0c73ea7ca1d339e4/src/bootstrap/builder.rs#L480 - maybe we need to change impl Step for Clippy to use suite_path?

Originally posted by @jyn514 in #103636 (comment)

[est31]

FTR this is also badly documented in clippy itself. You can't find it in the contributor instructions for example. It's also not printed by compiletest when it says "if you want to re-run the test do X" (that gives you a wrong command).

[jyn514]

@est31 you should report that on the clippy repo.

[est31]

@jyn514 fair. Done: rust-lang/rust-clippy#9825

[reez12g]

@rustbot claim

[jieyouxu]

Triage: clippy no longer uses compiletest (it uses ui_test now I think), but still uses TESTNAME.

[jieyouxu]

Clippy is a special snowflake and uses TESTNAME to tell it how to run individual tests instead of an argument to compiletest. Currently the only way to run individual tests is with TESTNAME=ice-6250 x test src/tools/clippy --bless. We should support x test src/tools/clippy/tests/ice-6250.rs as well.

#103266 will help simplify this a little by supporting the same test arguments as in-tree compiletest, but it won't solve suite paths automatically. Suite code lives around 51de238/src/bootstrap/builder.rs#L480 - maybe we need to change impl Step for Clippy to use suite_path?

IIRC suite_path is only really meant for compiletest, and this might be tricky because clippy isn't using compiletest.