Fix nix build to compilation error at Check Phase

- "panic_handler" should not be declared during test
- "main" and "std" are needed during test

src/main.rs

@@ -1,5 +1,5 @@
-#![no_main]
-#![no_std]
+#![cfg_attr(not(test), no_main)]
+#![cfg_attr(not(test), no_std)]
 
 use cortex_m_rt::entry;
 use log::info;
@@ -10,21 +10,22 @@ mod laser_diode;
 use device::{boot::bootup, log_setup, sys_timer};
 
 // If RTT is used, print panic info through RTT
-#[cfg(feature = "RTT")]
+#[cfg(all(feature = "RTT", not(test)))]
 use {
     core::panic::PanicInfo,
     rtt_target::rprintln,
 };
-#[cfg(feature = "RTT")]
+#[cfg(all(feature = "RTT", not(test)))]
 #[panic_handler]
 fn panic(info: &PanicInfo) -> ! {
     rprintln!("{}", info);
     loop {}
 }
 // Otherwise use panic halt
-#[cfg(not(feature = "RTT"))]
+#[cfg(all(not(feature = "RTT"), not(test)))]
 use panic_halt as _;
 
+#[cfg(not(test))]
 #[entry]
 fn main() -> ! {