Rollup merge of #95369 - jyn514:test-rustdoc, r=Mark-Simulacrum

Rollup merge of #95369 - jyn514:test-rustdoc, r=Mark-Simulacrum

Fix `x test src/librustdoc` with `download-rustc` enabled

The problem was two-fold:
- Bootstrap was hard-coding that unit tests should always run with stage1, not stage2, and
- It hard-coded the sysroot layout in stage1, which puts libLLVM.so in `lib/rustlib/` instead of just `lib/`.

This also takes the liberty of fixing `test src/librustdoc --no-doc`, which has been broken since it was first added. It would be nice at some point to unify this logic with other tests; I opened a Zulip thread: https://rust-lang.zulipchat.com/#narrow/stream/131828-t-compiler/topic/Inconsistency.20in.20.60x.20test.60

Fixes https://github.com/rust-lang/rust/issues/91071.