Remove uneccessary `std` bounds on many tests

Remove uneccessary `std` bounds on many tests

We never actually build with `#![no_std]` in tests as Rust does
not support it. Thus, many tests have spurious `std` feature gates
when they run just fine without them. Here we remove those gates,
though note that tests that depend on behavior elsewhere in the
codebase which is `std`-gated obviously need to retain their
feature gates.