I want to test a no_std lib using std, but I'm running into issues. I added `#![cfg_attr(not(test), no_std)]` to my lib, and now I'm getting this:
```
error[E0463]: can't find crate for `std`
  |
  = note: the `thumbv6m-none-eabi` target may not support the standard library
  = note: `std` is required by `critical_section` because it does not declare `#![no_std]`
  = help: consider building the standard library from source with `cargo build -Zbuild-std`
```