feat: make no_std conditional for testing

Wrap the `#![no_std]` attribute with `cfg_attr(not(test), no_std)` to allow
using the standard library when running tests.

Signed-off-by: Steven Lee <steven_lee@aspeedtech.com>

Author: stevenlee7189

src/lib.rs

@@ -1,6 +1,6 @@
 // Licensed under the Apache-2.0 license
 
-#![no_std]
+#![cfg_attr(not(test), no_std)]
 pub mod ecdsa;
 pub mod hace_controller;
 pub mod hash;