TESTNAME should be in CONTRIBUTING.md

TESTNAME is currently underdocumented. It is mentioned in the book but the book is very long and it explains stuff like how to use git and stuff. For me, as someone coming from rustc, who was accustomed to x.py test path/to/test working, TESTNAME was quite non-obvious. It's also not suggested when compiletest prints that a test has failed:

To only update this specific test, also pass `--test-args manual_let_else.rs`

If you try that flag you get:

$ cargo uitest --test-args manual_let_else.rs
error: Found argument '--test-args' which wasn't expected, or isn't valid in this context

And:

$ cargo uitest -- --test-args manual_let_else.rs
    Finished test [unoptimized + debuginfo] target(s) in 0.18s
     Running tests/compile-test.rs (target/debug/deps/compile_test-a3c7e3c02f9d451b)
error: Unrecognized option: 'test-args'
error: test failed, to rerun pass `--test compile-test`

In #8437 , I ended up running all tests each time which wasted a lot of time. I think a mention of TESTNAME definitely belongs into CONTRIBUTING.md.

cc https://github.com/rust-lang/rust/issues/104200

Start with CONTRIBUTING.md and its existing test-running instructions. Document how TESTNAME is used to run or update a specific test, taking the compiletest output and the cargo uitest examples in the issue as context. Done means a newcomer can find and use the focused-test workflow without running the full suite.