Rollup merge of #96303 - jyn514:improved-bootstrap-tests, r=Mark-Simulacrum

Improve bootstrap tests

- Don't checkout submodules in bootstrap tests

  This doesn't cause any tests to fail, and can greatly speed them up.

- Add a test for --exclude test::XXX

  I didn't know that the `test::` syntax was valid before, and it doesn't seem to be documented anywhere. Add a test so it doesn't regress accidentally, and as executable documentation.
  This also moves the `exclude` tests out of `dist`, to avoid assertion errors when the `cmd` passed to configure didn't match the `subcommand` used.

- Use run_build helper consistently across most bootstrap tests
  This is not super important to do, but the consistency is nice.

  I didn't change any tests that call `configure("dist")` and then override the subcommand - doing
that at all is pretty sketchy, but I don't want to mess with it while already doing a refactor.

Found while working on the "one call to Step for all paths" change mentioned in #95503 (comment), but independent of that work.

cc `@pietroalbini` for the `--exclude` test, git blame says you added support for it originally.

Author: matthiaskrgr