Compiler unit tests are never run in CI

I tried this code on a checkout branched from f5e2df741b4a9820a7579f0c8eccc951706a8782:

$ x test compiler/rustc_parse_format

I expected to see this happen: No warnings.

Instead, this happened:

Testing stage1 {rustc_parse_format} (aarch64-apple-darwin)
   Compiling rustc_index v0.0.0 (/Users/jyn/src/rust-upstream/compiler/rustc_index)
warning: extern crate `smallvec` is unused in crate `rustc_index`
 --> compiler/rustc_index/src/lib.rs:2:1
  |
2 | #![cfg_attr(all(feature = "nightly", test), feature(stmt_expr_attributes))]
  | ^
  |
  = help: remove the dependency or add `use smallvec as _;` to the crate root
  = note: requested on the command line with `-W unused-crate-dependencies`

This in itself isn't a big deal, but it seems bad that we aren't running tests that would have caught this ... I spot-checked the logs for the latest bors merge (https://github.com/rust-lang/rust/actions/runs/18907890296/job/53970647517) and couldn't find rustc_parse_format anywhere in them.

Start with the reported x test compiler/rustc_parse_format invocation and inspect the linked GitHub Actions bors-merge logs for how compiler unit tests are selected. Trace the CI test selection around compiler/rustc_parse_format and verify why it is absent. Done means the test is exercised in CI and the relevant warning or failure is covered by the resulting run.