Rollup merge of #109044 - thomcc:disallow-unstable-libtest, r=jyn514

Prevent stable `libtest` from supporting `-Zunstable-options`

Took a while for me to get around to this but seems trivial (unless I'm missing some reason this will break all our tests). Fixes #75526

Basically `libtest` already tries to handle this in https://github.com/rust-lang/rust/blob/501ad021b9a4fb2cd6a39e0302d22f169f6166b0/library/test/src/cli.rs#L310-L318

But that env var was not passed. I'm guessing at one point [this code](https://github.com/rust-lang/rust/blob/501ad021b9a4fb2cd6a39e0302d22f169f6166b0/src/bootstrap/compile.rs#L842-L844) (or a common ancestor) was used to compile the standard library/libtest, but that is no longer the case (or perhaps it never worked, I don't have time to go digging).

I don't love that this is a "allow unstable by default" situation, as it means things like [`rustc-build-sysroot`](https://github.com/RalfJung/rustc-build-sysroot) could accidentally get unstable (CC ````@RalfJung)```` even if this is fixed here, but it's consistent with what happens in `rustc_feature`, so... yeah.

This is user-facing after all, even if it's hard to imagine the outcome of that conversation being "lets continue allowing use of `-Zunstable-features` from stable rust" (especially since a `RUSTC_BOOTSTRAP=1`-shaped loophole remains)... I think it probably should get a vibe check in the t-libs meeting (and plausibly a relnote along the lines of "hey `cargo test -- -Zunstable-options --some --unstable --stuff=here` used to work on stable, that's been fixed, sorry").

I'll nominate it for that after CI comes up green (I've done a smoke check but don't know what (if anything) will need `bootstrap` to enable `RUSTC_BOOTSTRAP=1` when running tests)

r? ````@jyn514````

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