testing: remove nightly asm_const feature as it is already stablized

This feature is stablized in Rust 1.82.0: https://github.com/rust-lang/rust/pull/128570

Signed-off-by: Zhouqi Jiang <luojia@hust.edu.cn>

sbi-testing/src/lib.rs

@@ -2,7 +2,7 @@
 
 #![no_std]
 #![deny(warnings, missing_docs)]
-#![feature(naked_functions, asm_const)]
+#![feature(naked_functions)]
 
 mod thread;