rust: Remove --all-features flag from `cargo test` call in test_rust.sh.
We'd like to feature gate code that calls C from Rust, as a workaround
to several linker issues when running `cargo test` (#25386), and we
can't feature gate anything out of test code if `cargo test` is called
with `--all-features`.
* FIXES #26400: https://bugs.torproject.org/26400