Rollup merge of #134883 - Zalathar:shared-helpers, r=clubby789

bootstrap: Fix `./x check bootstrap` by moving `shared_helpers::tests`

Running `./x check bootstrap` currently doesn't work, because it builds the bootstrap shim binaries with `cfg(test)`, and those binaries can't find a `tests` submodule when they include `shared_helpers.rs` via `#[path]`.

This PR fixes that by taking the tests module and moving it to `super::tests::shared_helpers_tests` instead.

(The extra `tests` submodule prevents tidy from complaining about unit tests that aren't in a dedicated tests module.)

---

It would be nice to also run `./x check bootstrap compiletest` in CI, so that this and #134848 don't regress, but I didn't want to bundle that change with this fix.