`cargo test -- -Z unstable-options` is accepted by the stable libtest framework

[Nemo157]

I tried runnning

> cargo +stable test -- -Z unstable-options --format json

I expected to see this happen: an error message similar to

> cargo +stable test -Z unstable-options
error: the `-Z` flag is only accepted on the nightly channel of Cargo, but this is the `stable` channel
See https://doc.rust-lang.org/book/appendix-07-nightly-rust.html for more information about Rust release channels.

Instead, this happened: the tests ran and printed their output in json

Meta

> rustc +stable --version
rustc 1.45.2 (d3fb005a3 2020-07-31)

(mentioned in rust-lang/rustfmt#4228 (comment), cc #49359)

[ehuss]

See also #65770. There are multiple issues with how this is handled.