Auto merge of #96375 - jyn514:bootstrap-submodules, r=Mark-Simulacrum

Auto merge of #96375 - jyn514:bootstrap-submodules, r=Mark-Simulacrum

Fix running bootstrap tests on a fresh clone

In #96303, I changed the tests not to manage submodules, with the main
goal of avoiding a clone for llvm-project. Unfortunately, there are some tests
which depend on submodules - I didn't notice locally because they were already checked out for me,
and CI doesn't use submodule handling at all. Fresh clones, however, were impacted:
```
failures:

---- builder::tests::defaults::doc_default stdout ----
thread 'main' panicked at 'fs::read_dir(builder.src.join(&relative_path).join("redirects")) failed with No such file or directory (os error 2)', src/bootstrap/doc.rs:232:21
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

---- builder::tests::dist::dist_only_cross_host stdout ----
thread 'main' panicked at 'fs::read_to_string(&toml_file_name) failed with No such file or directory (os error 2)', src/bootstrap/lib.rs:1314:20
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
```

Try and get the best of both worlds by only checking out the submodules actually used in tests.