fix(libtest): Enable Instant on Emscripten targets `Instant::now()` works correctly on Emscripten since https://github.com/rust-lang/libc/pull/3962. All wasm-family targets with OS support can now handle instants. Improves #131738. ~~This changes the behavior of libtest on `unknown-unknown`/`unknown-none` wasm targets, but as far as I can see, libtest doesn't support them anyway. (Can anyone double-check?)~~ UPD: this patch no longer affects `unknown-unknown` targets. ``@rustbot`` label +A-libtest +T-testing-devex +O-emscripten +O-wasm -needs-triage