bootstrap: Describe build_steps::test for 2024

The description was most accurate when it was still called rustbuild,
and I presume indeed did mostly run in CI. It has become something more
so try to describe it better for current-day usage.

src/bootstrap/src/core/build_steps/test.rs

@@ -1,7 +1,7 @@
-//! Implementation of the test-related targets of the build system.
+//! Build-and-run steps for `./x.py test` test fixtures
 //!
-//! This file implements the various regression test suites that we execute on
-//! our CI.
+//! `./x.py test` (aka [`Kind::Test`]) is currently allowed to reach build steps in other modules.
+//! However, this contains ~all test parts we expect people to be able to build and run locally.
 
 use std::env;
 use std::ffi::OsStr;