compiletest: Add testing for lint machinery at runtime

[clubby789]

While implementing #157707, I noticed that we don't have 'runtime' tests for the diagnostic machinery (i.e. we test the derive macros themselves, but not their output when emitted). This introduces a new way to test this machinery.

• Add //@ aux-lint: file.rs, which builds a lint library in auxiliary/file.rs
• Lint libraries link with rustc_private, and export a extern "C" fn register_lints(...)
• A new compiletest-lint-driver wraps rustc and installs the provided lints when building the test

Included is a simple demo lint for example

AI Disclosure: I used an LLM to plan out the broad structure of the machinery and identify issues; all code in this PR is written by hand