Rollup merge of #136353 - purplesyringa:libtest-instant-wasm, r=Mark-Simulacrum

fix(libtest): Enable Instant on Emscripten targets

`Instant::now()` works correctly on Emscripten since rust-lang/libc#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

Author: workingjubilee