Fixed usage of String in a #[no_std] environment in ascii_str::tests.

Author: Zenithsiz

src/ascii_str.rs

@@ -1184,11 +1184,10 @@ mod tests {
         let ascii_str = arr.as_ref().into();
         let mut mut_arr = arr; // Note: We need a second copy to prevent overlapping mutable borrows.
         let mut_ascii_str = mut_arr.as_mut().into();
-        let mut_arr_mut_ref: &mut [AsciiChar] = &mut [AsciiChar::A];
-        let mut string = "A".to_string();
-        let mut string2 = "A".to_string();
-        let string_mut = string.as_mut_str();
-        let string_mut_bytes = unsafe { string2.as_bytes_mut() }; // SAFETY: We don't modify it
+		let mut_arr_mut_ref: &mut [AsciiChar] = &mut [AsciiChar::A];
+		let mut string_bytes = [b'A'];
+        let string_mut = unsafe { std::str::from_utf8_unchecked_mut(&mut string_bytes) }; // SAFETY: 'A' is a valid string.
+        let string_mut_bytes: &mut [u8] = &mut [b'A'];
 
         // Note: This is a trick because `rustfmt` doesn't support
         //       attributes on blocks yet.